Add support for WITH clause about dsql

[romaninsh]

Further reading:

• http://guilhembichot.blogspot.co.uk/2013/11/with-recursive-and-mysql.html?m=1
• https://en.wikipedia.org/wiki/Hierarchical_and_recursive_queries_in_SQL#Common_table_expression
• http://www.sqlite.org/lang_with.html

[romaninsh]

@tasso85 , thanks for suggesting the feature. I have expanded your definition to be more specific about implementation, in case someone might decide to implement it.

[DarkSide666]

This syntax is specific for DB engine, so it should be implemented somewhere in extended engine-specific classes.

[tasso85]

It doesn't seem specific to a DB engine to me:

https://www.postgresql.org/docs/9.1/static/queries-with.html
https://msdn.microsoft.com/en-us/library/ms175972.aspx
https://mariadb.com/kb/en/mariadb/with/
http://mysqlserverteam.com/mysql-8-0-labs-recursive-common-table-expressions-in-mysql-ctes/

they all seem to use the same syntax (even though it is still not implemented in MySQL, but is available in MariaDB)

From Wikipedia:

Common table expressions are supported by Teradata, DB2, Firebird,[14] Microsoft SQL Server, Oracle (with recursion since 11g release 2), PostgreSQL (since 8.4), MariaDB (since 10.2), SQLite (since 3.8.3), HyperSQL and H2 (experimental).[15] Oracle calls CTEs "subquery factoring".[16] Though MySQL does not support common table expressions there exists a demonstration of WITH RECURSIVE emulation[17] using stored procedures.

[DarkSide666]

MySQL 8.x now also supports WITH.