Support set variables

[cprieto]

Support set variables for certain SQL dialects.

• MySQL, SET [GLOBAL | SESSION] [@]variable = value;
• PostgreSQL, SET [LOCAL | SESSION] variable [=|TO] value

SQL Server cannot set a variable without declaring it first:

DECLARE @variable INT;
SET @variable = 1;

In MySQL, the prefix @ is optional while in SQL Server is required.

SQLite does not support setting variables.