Nested query  

  • Nested query:

    • A nested query is a SELECT query where the WHERE clause contains subquery.

  • Schematically:   what a nested query looks link 

     Nested query:

   SELECT ...
   FROM   ...
   WHERE  ... (SELECT ...    /* Subquery */   
	       FROM ... 
	       WHERE ...)

     

     

  How deep can you nest queries ?  

  • In theory, nesting can be arbitrarily deep: 

       SELECT ...
   FROM   ...
   WHERE  ... (SELECT ... 
	       FROM ... 
	       WHERE ... (SELECT ...
			  FROM ...
			  WHERE ... ( ...
			              ...
                                    )
                          )
               )

  • In practice, depending on your DBMS system, the number of nesting levels is limited (and different for each system)

    • According to click here, MySQL can have 60 levels of nested selects.