Руководство для начинающих по SQL

Sample table

В футбольном тренировочном лагере тренер традиционно начинает с того, что показывает игрокам футбольный мяч и говорит: «Это — футбольный мяч». В случае SQL подобный базовый объект — это таблица:

TABLE
          [1]              [2]              [3]
       +-----------------+----------------+----------------+
 Стрк1 |  Стрк1, Стлб1   |  Стрк1, Стлб2  |  Стрк1, Стлб3  |
       +-----------------+----------------+----------------+
 Стрк2 |  Стрк2, Стлб1   | Стрк2, Стлб2   |  Стрк2, Стлб3  |
       +-----------------+----------------+----------------+
 Стрк3 |  Стрк3, Стлб1   |  Стрк3, Стлб2  |  Стрк3, Стлб3  |
       +-----------------+----------------+----------------+

But the labels are misleading – one usually doesn’t identify rows and columns by their ordinal positions, one prefers to pick out specific items by their contents. In that spirit, this is a table:

MODULES

+-----------------+------+---------------------+
| NAME            | SIZE | PURPOSE             |
+-----------------+------+---------------------+
| box             | 1432 | Управление БД       |
| clock           |  188 | Секунды             |
| crypto          |    4 | Криптография        |
+-----------------+------+---------------------+

So one does not use longitude/latitude navigation by talking about «Row#2 Column #2», one uses the contents of the Name column and the name of the Size column by talking about «the size, where the name is „clock“». To be more exact, this is what one says:

SELECT size FROM modules WHERE name = 'clock';

If you’re familiar with Tarantool’s architecture – and ideally you read about that before coming to this chapter – then you know that there is a NoSQL way to get the same thing:

box.space.MODULES:select()[2][2]

Well, you can do that. One of the advantages of Tarantool is that if you can get data via an SQL statement, then you can get the same data via a NoSQL request. But the reverse is not true, because not all NoSQL tuple sets are definable as SQL tables. These restrictions apply for SQL that do not apply for NoSQL:
1. Every column must have a name.
2. Every column should have a scalar type (Tarantool is relaxed about which particular scalar type you can have, but there is no way to index and search arrays, tables within tables, or what MessagePack calls «maps».)

Tarantool/NoSQL’s «format» clause causes the same restrictions.

So an SQL «table» is a NoSQL «tuple set with format restrictions», an SQL «row» is a NoSQL «tuple», an SQL «column» is a NoSQL «list of fields within a tuple set».

Creating a table

This is how to create the modules table:

CREATE TABLE modules (name STRING, size INTEGER, purpose STRING, PRIMARY KEY (name));

The words that are IN CAPITAL LETTERS are «keywords» (although it is only a convention in this manual that keywords are in capital letters, in practice many programmers prefer to avoid shouting). A keyword has meaning for the SQL parser so many keywords are reserved, they cannot be used as names unless they are enclosed inside quotation marks.

The word «modules» is a «table name», and the words «name» and «size» and «purpose» are «column names». All tables and all columns must have names.

The words «STRING» and «INTEGER» are «data types». STRING means «the contents should be characters, the length is indefinite, the equivalent NoSQL type is „string““». INTEGER means «the contents should be numbers without decimal points, the equivalent NoSQL type is „integer“». Tarantool supports other data types but this section’s example table has data types from the two main groups, namely, data types for numbers and data types for strings.

The final clause, PRIMARY KEY (name), means that the name column is the main column used to identify the row.

Nulls

Frequently it is necessary, at least temporarily, that a column value should be NULL. Typical situations are: the value is unknown, or the value is not applicable. For example, you might make a module as a placeholder but you don’t want to say its size or purpose. If such things are possible, the column is «nullable». The example table’s name column cannot contain nulls, and it could be defined explicitly as «name STRING NOT NULL», but in this case that’s unnecessary – a column defined as PRIMARY KEY is automatically NOT NULL.

Is a NULL in SQL the same thing as a nil in Lua? No, but it is close enough that there will be confusion. When nil means «unknown» or «inapplicable», yes. But when nil means «nonexistent» or «type is nil», no. NULL is a value, it has a data type because it is inside a column which is defined with that data type.

Creating an index

This is how to create indexes for the modules table:

CREATE INDEX size ON modules (size);
CREATE UNIQUE INDEX purpose ON modules (purpose);

There is no need to create an index on the name column, because Tarantool creates an index automatically when it sees a PRIMARY KEY clause in the CREATE TABLE statement. In fact there is no need to create indexes on the size or purpose columns either – if indexes don’t exist, then it is still possible to use the columns for searches. Typically people create non-primary indexes, also called secondary indexes, when it becomes clear that the table will grow large and searches will be frequent, because searching with an index is generally much faster than searching without an index.

Another use for indexes is to enforce uniqueness. When an index is created with CREATE UNIQUE INDEX for the purpose column, it is not possible to have duplicate values in that column.

Data change

Putting data into a table is called «inserting». Changing data is called «updating». Removing data is called «deleting». Together, the three SQL statements INSERT plus UPDATE plus DELETE are the three main «data-change» statements.

This is how to insert, update, and delete a row in the modules table:

INSERT INTO modules VALUES ('json', 14, 'format functions for JSON');
UPDATE modules SET size = 15 WHERE name = 'json';
DELETE FROM modules WHERE name = 'json';

The corresponding non-SQL Tarantool requests would be:

box.space.MODULES:insert{'json', 14, 'format functions for JSON'}
box.space.MODULES:update('json', {{'=', 2, 15}})
box.space.MODULES:delete{'json'}

This is how one would populate the table with the values that was shown earlier:

INSERT INTO modules VALUES ('box', 1432, 'Database Management');
INSERT INTO modules VALUES ('clock', 188, 'Seconds');
INSERT INTO modules VALUES ('crypto', 4, 'Cryptography');

Constraints

Some data-change statements are illegal due to something in the table’s definition. This is called «constraining what can be done». Some types of constraints have already been shown …

NOT NULL – if a column is defined with a NOT NULL clause, it is illegal to put NULL into it. A primary-key column is automatically NOT NULL.

UNIQUE – if a column has a UNIQUE index, it is illegal to put a duplicate into it. A primary-key column automatically has a UNIQUE index.

Допустимые значения — если столбец по определению содержит тип данных INTEGER, в него запрещено помещать нечисловые значения. В целом, недопустимо любое значение, которое не соответствует типу данных, указанному в определении. Некоторые системы управления базами данных относительно гибки и скорее примут некорректные значения, чем отклонят их. Tarantool в этом плане более строгий.

Now, here are other types of constraints …

CHECK – a table description can have a clause «CHECK (conditional expression)». For example, if the CREATE TABLE modules statement looked like this:

CREATE TABLE modules (name STRING,
                      size INTEGER,
                      purpose STRING,
                      PRIMARY KEY (name),
                      CHECK (size > 0));

then this INSERT statement would be illegal:
INSERT INTO modules VALUES ('box', 0, 'The Database Kernel');
because there is a CHECK constraint saying that the second column, the size column, cannot contain a value which is less than or equal to zero. Try this instead:
INSERT INTO modules VALUES ('box', 1, 'The Database Kernel');

FOREIGN KEY – a table description can have a clause «FOREIGN KEY (column-list) REFERENCES table (column-list)». For example, if there is a new table «submodules» which in a way depends on the modules table, it can be defined like this:

CREATE TABLE submodules (name STRING,
                         module_name STRING,
                         size INTEGER,
                         purpose STRING,
                         PRIMARY KEY (name),
                         FOREIGN KEY (module_name) REFERENCES
                         modules (name));

Now try to insert a new row into this submodules table:

INSERT INTO submodules VALUES
  ('space', 'Box', 10000, 'insert etc.');

The insert will fail because the second column (module_name) refers to the name column in the modules table, and the name column in the modules table does not contain „Box“. However, it does contain „box“. By default searches in Tarantool’s SQL use a binary collation. This will work:

INSERT INTO submodules
  VALUES ('space', 'box', 10000, 'insert etc.');

Теперь попробуем удалить соответствующую строку из таблицы modules:

DELETE FROM modules WHERE name = 'box';

The delete will fail because the second column (module_name) in the submodules table refers to the name column in the modules table, and the name column in the modules table would not contain „box“ if the delete succeeded. So the FOREIGN KEY constraint affects both the table which contains the FOREIGN KEY clause and the table that the FOREIGN KEY clause refers to.

The constraints in a table’s definition – NOT NULL, UNIQUE, data domain, CHECK, and FOREIGN KEY – are guarantors of the database’s integrity. It is important that they are fixed and well-defined parts of the definition, and hard to bypass with SQL. This is often seen as a difference between SQL and NoSQL – SQL emphasizes law and order, NoSQL emphasizes freedom and making your own rules.

Table relationships

Think about the two tables that have been discussed so far:

CREATE TABLE modules (name STRING,
                      size INTEGER,
                       purpose STRING,
                       PRIMARY KEY (name),
                       CHECK (size > 0));

CREATE TABLE submodules (name STRING,
                         module_name STRING,
                         size INTEGER,
                         purpose STRING,
                         PRIMARY KEY (name),
                         FOREIGN KEY (module_name) REFERENCES
                         modules (name));

Because of the FOREIGN KEYS clause in the submodules table, there is clearly a many-to-one relationship:
submodules –>> modules
that is, every submodules row must refer to one (and only one) modules row, while every modules row can be referred to in zero or more submodules rows.

Table relationships are important, but beware: do not trust anyone who tells you that databases made with SQL are relational «because there are relationships between tables». That is wrong, as will be clear in the discussion about what makes a database relational, later.

Selecting with WHERE

SET SESSION "sql_seq_scan" = true;

We gave a simple example of a SELECT statement earlier:

SELECT size FROM modules WHERE name = 'clock';

The clause «WHERE name = „clock“» is legal in other statements – it is in examples with UPDATE and DELETE – but here the only examples will be with SELECT.

The first variation is that the WHERE clause does not have to be specified at all, it is optional. So this statement would return all rows:

SELECT size FROM modules;

The second variation is that the comparison operator does not have to be „=“, it can be anything that makes sense: „>“ or „>=“ or „<“ or „<=“, or „LIKE“ which is an operator that works with strings that may contain wildcard characters „_“ meaning „match any one character“ or „%“ meaning „match any zero or one or many characters“. These are legal statements which return all rows:

SELECT size FROM modules WHERE name >= '';
SELECT size FROM modules WHERE name LIKE '%';

The third variation is that IS [NOT] NULL is a special condition. Remembering that the NULL value can mean «it is unknown what the value should be», and supposing that in some row the size is NULL, then the condition «size > 10» is not certainly true and it is not certainly false, so it is evaluated as «unknown». Ordinarily the application of a WHERE clause filters out both false and unknown results. So when searching for NULL, say IS NULL; when searching anything that is not NULL, say IS NOT NULL. This statement will return all rows because (due to the definition) there are no NULLs in the name column:

SELECT size FROM modules WHERE name IS NOT NULL;

The fourth variation is that conditions can be combined with AND / OR, and negated with NOT.

So this statement would return all rows (the first condition is false but the second condition is true, and OR means «return true if either condition is true»):

SELECT size
FROM modules
WHERE name = 'wombat' OR size IS NOT NULL;

Selecting with a select list

Yet again, here is a simple example of a SELECT statement:

SELECT size FROM modules WHERE name = 'clock';

The words between SELECT and FROM are the select list. In this case, the select list is just one word: size. Formally it means that the desire is to return the size values, and technically the name for picking a particular column is called «projection».

The first variation is that one can specify any column in any order:

SELECT name, purpose, size FROM modules;

The second variation is that one can specify an expression, it does not have to be a column name, it does not even have to include a column name. The common expression operators for numbers are the arithmetic operators + - / *; the common expression operator for strings is the concatenation operator ||. For example this statement will return 8, „XY“:

SELECT size * 2, 'X' || 'Y' FROM modules WHERE size = 4;

The third variation is that one can add a clause [AS name] after every expression, so that in the return the column titles will make sense. This is especially important when a title might otherwise be ambiguous or meaningless. For example this statement will return 8, „XY“ as before

SELECT size * 2 AS double_size, 'X' || 'Y' AS concatenated_literals  FROM modules
  WHERE size = 4;

but displayed as a table the result will look like

+----------------+------------------------+
| DOUBLE_SIZE    | CONCATENATED_LITERALS  |
+----------------+------------------------+
|              8 | XY                     |
+----------------+------------------------+

Selecting with a select list with asterisk

Instead of listing columns in a select list, one can just say '*'. For example

SELECT * FROM modules;

This is the same thing as

SELECT name, size, purpose FROM modules;

Selecting with "*" saves time for the writer, but it is unclear to a reader who has not memorized what the column names are. Also it is unstable, because there is a way to change a table’s definition (the ALTER statement, which is an advanced topic). Nevertheless, although it might be bad to use it for production, it is handy to use it for introduction, so "*" will appear in some following examples.

Select with subqueries

Remember that there is a modules table and there is a submodules table. Suppose that there is a desire to list the submodules that refer to modules for which the purpose is X. That is, this involves a search of one table using a value in another table. This can be done by enclosing «(SELECT …)» within the WHERE clause. For example:

SELECT name FROM submodules
WHERE module_name =
    (SELECT name FROM modules WHERE purpose LIKE '%Database%');

Subqueries are also useful in the select list, when one wishes to combine information from more than one table. For example this statement will display submodules rows but will include values that come from the modules table:

SELECT name AS submodules_name,
    (SELECT purpose FROM modules
     WHERE modules.name = submodules.module_name)
     AS modules_purpose,
    purpose AS submodules_purpose
FROM submodules;

Whoa. What are «modules.name» and «submodules.name»? Whenever you see «x . y» you are looking at a «qualified column name», and the first part is a table identifier, the second part is a column identifier. It is always legal to use qualified column names, but until now it has not been necessary. Now it is necessary, or at least it is a good idea, because both tables have a column named «name».

Результат будет выглядеть следующим образом:

+-------------------+------------------------+--------------------+
| SUBMODULES_NAME   | MODULES_PURPOSE        | SUBMODULES_PURPOSE |
+-------------------+------------------------+--------------------+
| space             | Управление БД          | Вставка ...        |
+-------------------+------------------------+--------------------+

Perhaps you have read somewhere that SQL stands for «Structured Query Language». That is not true any more. But it is true that the query syntax allows for a structural component, namely the subquery, and that was the original idea. However, there is a different way to combine tables – with joins instead of subqueries.

Select with Cartesian join

Until now only «FROM modules» or «FROM submodules» was used in SELECT statements. What if there was more than one table in the FROM clause? For example

SELECT * FROM modules, submodules;

or

SELECT * FROM modules JOIN submodules;

That is legal. Usually it is not what you want, but it is a learning aid. The result will be:

{ столбцы таблицы modules }         { столбцы таблицы submodules }
+--------+------+---------------------+-------+-------------+-------+-------------+
| NAME   | SIZE | PURPOSE             | NAME  | MODULE_NAME | SIZE  | PURPOSE     |
+--------+------+---------------------+-------+-------------+-------+-------------+
| box    | 1432 | Управление БД       | space | box         | 10000 | Вставка ... |
| clock  |  188 | Секунды             | space | box         | 10000 | Вставка ... |
| crypto |    4 | Криптография        | space | box         | 10000 | Вставка ... |
+--------+------+---------------------+-------+-------------+-------+-------------+

It is not an error. The meaning of this type of join is «combine every row in table-1 with every row in table-2». It did not specify what the relationship should be, so the result has everything, even when the submodule has nothing to do with the module.

It is handy to look at the above result, called a «Cartesian join» result, to see what would really be desirable. Probably for this case the row that actually makes sense is the one where the modules.name = submodules.module_name, and it’s better to make that clear in both the select list and the WHERE clause, thus:

SELECT modules.name AS modules_name,
       modules.size AS modules_size,
       modules.purpose AS modules_purpose,
       submodules.name,
       module_name,
       submodules.size,
       submodules.purpose
FROM modules, submodules
WHERE modules.name = submodules.module_name;

Мы получим следующий результат:

+----------+-----------+------------+--------+---------+-------+-------------+
| MODULES_ |  MODULES_ | MODULES_   | NAME   | MODULE_ | SIZE  | PURPOSE     |
| NAME     |  SIZE     | PURPOSE    |        | NAME    |       |             |
+----------+-----------+--------- --+--------+---------+-------+-------------+
| box      |      1432 | Управление | space  | box     | 10000 | Вставка ... |
|          |           | БД         |        |         |       |             |
+----------+-----------+------------+--------+---------+-------+-------------+

In other words, you can specify a Cartesian join in the FROM clause, then you can filter out the irrelevant rows in the WHERE clause, and then you can rename columns in the select list. This is fine, and every SQL DBMS supports this. But it is worrisome that the number of rows in a Cartesian join is always (number of rows in first table multiplied by number of rows in second table), which means that conceptually you are often filtering in a large set of rows.

It is good to start by looking at Cartesian joins because they show the concept. Many people, though, prefer to use different syntaxes for joins because they look better or clearer. So now those alternatives will be shown.

Select with join with ON clause

The ON clause would have the same comparisons as the WHERE clause that was illustrated for the previous section, but the use of different syntax would be making it clear «this is for the sake of the join». Readers can see at a glance that it is, in concept at least, an initial step before the result rows are filtered. For example this

SELECT * FROM modules JOIN submodules
  ON (modules.name = submodules.module_name);

is the same as

SELECT * FROM modules, submodules
  WHERE modules.name = submodules.module_name;

Select with join with USING clause

The USING clause would take advantage of names that are held in common between the two tables, with the assumption that the intent is to match those columns with „=“ comparisons. For example,

SELECT * FROM modules JOIN submodules USING (name);

has the same effect as

SELECT * FROM modules JOIN submodules WHERE modules.name = submodules.name;

If the table had been created with a plan in advance to use USING clauses, that would save time. But that did not happen. So, although the above example «works», the results will not be sensible.

Select with natural join

A natural join would take advantage of names that are held in common between the two tables, and would do the filtering automatically based on that knowledge, and throw away duplicate columns.

If the table had been created with a plan in advance to use natural joins, that would be very handy. But that did not happen. So, although the following example «works», the results won’t be sensible.

SELECT * FROM modules NATURAL JOIN submodules;

Result: nothing, because modules.name does not match submodules.name, and so on And even if there had been a result, it would only have included four columns: name, module_name, size, purpose.

Select with left join

Now what if there is a desire to join modules to submodules, but it’s necessary to be sure that all the modules are found? In other words, suppose the requirement is to get modules even if the condition submodules.module_name = modules.name is not true, because the module has no submodules.

When that is the requirement, the type of join is an «outer join» (as opposed to the type that has been used so far which is an «inner join»). Specifically the format will be LEFT [OUTER] JOIN because the main table, modules, is on the left. For example:

SELECT *
FROM modules LEFT JOIN submodules
ON modules.name = submodules.module_name;

which returns:

{ столбцы таблицы modules    }         { столбцы таблицы submodules    }
+--------+------+---------------------+-------+-------------+-------+-------------+
| NAME   | SIZE | PURPOSE             | NAME  | MODULE_NAME | SIZE  | PURPOSE     |
+--------+------+---------------------+-------+-------------+-------+-------------+
| box    | 1432 | Управление БД       | space | box         | 10000 | Вставка ... |
| clock  |  188 | Секунды             | NULL  | NULL        | NULL  | NULL        |
| crypto |    4 | Криптография        | NULL  | NULL        | NULL  | NULL        |
+--------+------+---------------------+-------+-------------+-------+-------------+

Thus, for the submodules of the clock module and the submodules of the crypto module – which do not exist – there are NULLs in every column.

Select with functions

A function can take any expression, including an expression that contains another function, and return a scalar value. There are many such functions. Here will be a description of only one, SUBSTR, which returns a substring of a string.

Format: SUBSTR(input-string, start-with [, length])

Description: SUBSTR takes input-string, eliminates any characters before start-with, eliminates any characters after (start-with plus length), and returns the result.

Example: SUBSTR('abcdef', 2, 3) returns „bcd“.

Select with aggregation, GROUP BY, and HAVING

Remember that the modules table looks like this:

MODULES

+-----------------+------+---------------------+
| NAME            | SIZE | PURPOSE             |
+-----------------+------+---------------------+
| box             | 1432 | Управление БД       |
| clock           |  188 | Секунды             |
| crypto          |    4 | Криптография        |
+-----------------+------+---------------------+

Suppose that there is no need to know all the individual size values, all that is important is their aggregation, that is, take the attributes of the collection. SQL allows aggregation functions including: AVG (average), SUM, MIN (minimum), MAX (maximum), and COUNT. For example

SELECT AVG(size), SUM(size), MIN(size), MAX(size), COUNT(size) FROM modules;

Результат будет выглядеть следующим образом:

+-----------+-----------+-----------+-----------+-----------+
| СТОЛБЕЦ1  | СТОЛБЕЦ2  | СТОЛБЕЦ3  | СТОЛБЕЦ4  | СТОЛБЕЦ5  |
+-----------+-----------+-----------+-----------+-----------|
|       541 |      1624 |         4 |      1432 |         3 |
+-----------+-----------+-----------+-----------+-----------+

Suppose that the requirement is aggregations, but aggregations of rows that have some common characteristic. Supposing further, the rows should be divided into two groups, the ones whose names begin with „b“ and the ones whose names begin with „c“. This can be done by adding a clause [GROUP BY expression]. For example,

SELECT SUBSTR(name, 1, 1), AVG(size), SUM(size), MIN(size), MAX(size), COUNT(size)
FROM modules
GROUP BY SUBSTR(name, 1, 1);

Результат будет выглядеть следующим образом:

+------------+--------------+-----------+-----------+-----------+-------------+
| СТОЛБЕЦ1   | СТОЛБЕЦ2     | СТОЛБЕЦ3  | СТОЛБЕЦ4  | СТОЛБЕЦ5  | СТОЛБЕЦ6    |
+------------+--------------+-----------+-----------+-----------+-------------+
| b          |         1432 |      1432 |      1432 |      1432 |           1 |
| c          |           96 |       192 |         4 |       188 |           2 |
+------------+--------------+-----------+-----------+-----------+-------------+

Select with common table expression

It is possible to define a temporary (viewed) table within a statement, usually within a SELECT statement, using a WITH clause. For example:

WITH tmp_table AS (SELECT x1 FROM t1) SELECT * FROM tmp_table;

Select with order, limit, and offset clauses

So far, tor every search in the modules table, the rows have come out in alphabetical order by name: „box“, then „clock“, then „crypto“. However, to really be sure about the order, or to ask for a different order, it is necessary to be explicit and add a clause: ORDER BY column-name [ASC|DESC]. (ASC stands for ASCending, DESC stands for DESCending.) For example:

SELECT * FROM modules ORDER BY name DESC;

The result will be the usual rows, in descending alphabetical order: „crypto“ then „clock“ then „box“.

After the ORDER BY clause there can be a clause LIMIT n, where n is the maximum number of rows to retrieve. For example:

SELECT * FROM modules ORDER BY name DESC LIMIT 2;

The result will be the first two rows, „crypto“ and „clock“.

After the ORDER BY clause and the LIMIT clause there can be a clause OFFSET n, where n is the row to start with. The first offset is 0. For example:

SELECT * FROM modules ORDER BY name DESC LIMIT 2 OFFSET 2;

The result will be the third row, „box“.

Views

A view is a canned SELECT. If you have a complex SELECT that you want to run frequently, create a view and then do a simple SELECT on the view. For example:

CREATE VIEW v AS SELECT size, (size *5) AS size_times_5
FROM modules
GROUP BY size, name
ORDER BY size_times_5;
SELECT * FROM v;

Transactions

Tarantool has a «Write Ahead Log» (WAL). Effects of data-change statements are logged before they are permanently stored on disk. This is a reason that, although entire databases can be stored in temporary memory, they are not vulnerable in case of power failure.

Tarantool supports commits and rollbacks. In effect, asking for a commit means asking for all the recent data-change statements, since a transaction began, to become permanent. In effect, asking for a rollback means asking for all the recent data-change statements, since a transaction began, to be cancelled.

For example, consider these statements:

CREATE TABLE things (remark STRING, PRIMARY KEY (remark));
START TRANSACTION;
INSERT INTO things VALUES ('A');
COMMIT;
START TRANSACTION;
INSERT INTO things VALUES ('B');
ROLLBACK;
SELECT * FROM things;

The result will be: one row, containing „A“. The ROLLBACK cancelled the second INSERT statement, but did not cancel the first one, because it had already been committed.

Ordinarily every statement is automatically committed.

After START TRANSACTION, statements are not automatically committed – Tarantool considers that a transaction is now «active», until the transaction ends with a COMMIT statement or a ROLLBACK statement. While a transaction is active, all statements are legal except another START TRANSACTION.

Implementing Tarantool’s SQL On Top of NoSQL

Tarantool’s SQL data is the same as Tarantool’s NoSQL data. When you create a table or an index with SQL, you are creating a space or an index in NoSQL. For example:

CREATE TABLE things (remark STRING, PRIMARY KEY (remark));
INSERT INTO things VALUES ('X');

is somewhat similar to

box.schema.space.create('THINGS',
{
    format = {
              [1] = {["name"] = "REMARK", ["type"] = "string"}
              }
})
box.space.THINGS:create_index('pk_unnamed_THINGS_1',{unique=true,parts={1,'string'}})
box.space.THINGS:insert{'X'}

Therefore you can take advantage of Tarantool’s NoSQL features even though your primary language is SQL. Here are some possibilities.

(1) NoSQL applications written in one of the connector languages may be slightly faster than SQL applications because SQL statements may require more parsing and may be translated to NoSQL requests.

(2) You can write stored procedures in Lua, combining Lua loop-control and Lua library-access statements with SQL statements. These routines are executed on the server, which is the principal advantage of pure-SQL stored procedures.

(3) There are some options that are implemented in NoSQL that are not (yet) implemented in SQL. For example you can use NoSQL to change an index option, and to deny access to users named „guest“.

(4) System spaces such as _space and _index can be accessed with SQL SELECT statements. This is not quite the same as an information_schema, but it does mean that you can use SQL to access the database’s metadata catalog.

Fields in NoSQL spaces can be accessed with SQL if and only if they are scalar and are defined in format clauses. Indexes of NoSQL spaces will be used with SQL if and only if they are TREE indexes.

Relational databases

Edgar F. Codd, the person most responsible for researching and explaining relational database concepts, listed the main criteria as (Codd’s 12 rules).

Although Tarantool is not advertised as «relational», Tarantool comes with a claim that it complies with these rules, with the following caveats and exceptions …

The rules state that all data must be viewable as relations. A Tarantool SQL table is a relation. However, it is possible to have duplicate values in SQL tables and it is possible to have an implicit ordering. Those characteristics are not allowed for true relations.

The rules state that there must be a dynamic online catalog. Tarantool has one but some metadata is missing from it.

The rules state that the data language must support authorization. Tarantool’s SQL does not. Authorization occurs via NoSQL requests.

The rules require that data must be physically independent (from underlying storage changes) and logically independent (from application program changes). So far there is not enough experience to make this guarantee.

The rules require certain types of updatable views. Tarantool’s views are not updatable.

The rules state that it should be impossible to use a low-level language to bypass integrity as defined in the relational-level language. In Tarantool’s case, this is not true, for example one can execute a request with Tarantool’s NoSQL to violate a foreign-key constraint that was defined with Tarantool’s SQL.

To learn more about SQL in Tarantool, check the reference.