mysql dynamic query in stored procedure

Question

i am creating a dynamic query in stored procedure. my stored procedure is as follows:

CREATE PROCEDURE `test1`(IN tab_name VARCHAR(40),IN w_team VARCHAR(40))         


        

Answer 1

Try this..

CREATE PROCEDURE `test1`(IN tab_name VARCHAR(40),IN w_team VARCHAR(40))
BEGIN
SET @t1 =CONCAT("SELECT * FROM ",tab_name," where team='",w_team,"'");
 PREPARE stmt3 FROM @t1;
 EXECUTE stmt3;
 DEALLOCATE PREPARE stmt3;
END

You are missing quotes around w_team variable..

you should print the statement that dynamically build so you can just copy printed statement and try so you can easily find this kind of problem.

select @t1 will print the statment that build dynamically..

Answer 2

You missed the quotes in WHERE clause.

Try like this:

SET @t1 =CONCAT("SELECT * FROM ",tab_name," where team='",w_team,"'");

Explanation:

Query from your code would be like:

SELECT * FROM Test where team=SPA

And we changed it to:

SELECT * FROM Test where team='SPA'

Answer 3

Error Code: 1054. Unknown column 'SPA' in 'where clause'

This happens when you do not enclose input string within quotes, and SQL engine tries to identify it as a column in the table being queried. But it fails as it can't find it.

But what happens when it finds such column?
It fetches results when it finds some matches on the column values.
Obviously this is not what one was expecting.

How to overcome this? Use Prepared Statements with dynamic input values.

You can use placeholders like ? in stored procedures too on dynamic input values to use with Prepared Statements. The engine will handle escape characters and other string values when assigned to or compared within SQL expressions.

You just need to re-assign procedure inputs to one or more session variables, as required.

Example on your procedure:

CREATE PROCEDURE `test1`( IN tab_name VARCHAR(40), IN w_team VARCHAR(40) )
BEGIN
  SET @t1 = CONCAT( 'SELECT * FROM ', tab_name, ' where team = ?' ); -- <-- placeholder
  SET @w_team := w_team;

  PREPARE stmt3 FROM @t1;
  EXECUTE stmt3 USING @w_team; -- <-- input for placeholder
  DEALLOCATE PREPARE stmt3;
END;