How to Retrieve Dynamic Query Results into OUT Parameters in MySQL Stored Procedures?

Dynamic Query Execution and Retrieving Result into a Variable in MySQL Stored Procedure

In MySQL, you can generate dynamic queries within stored procedures to handle complex and flexible data operations. This article will guide you through retrieving the result of a dynamic query into an OUT parameter.

Consider the following stored procedure:

CREATE PROCEDURE searchInvoice(
  OUT numOfRecords INT
)
BEGIN
  DECLARE query1 TEXT; 
  DECLARE query2 TEXT; 

  SET query1 = 'SELECT COUNT(*) AS bla FROM bla bla bla.....'; 
  SET query2 = 'SELECT * FROM bla bla bla....';

  -- Dynamically generate the rest of both queries based on IN parameters.

  -- Here, you want to assign the output of query1 to numOfRecords.

  SET @Sql = query2;        
  PREPARE STMT FROM @Sql; 
  EXECUTE STMT; 
  DEALLOCATE PREPARE STMT;

  -- PHP can access the output of query2 here.
END

To retrieve the output of query1 into the OUT parameter numOfRecords, you can use the following technique:

SET @outVar = @queryResult;

For example:

SET @numOfRecords = @query1Result;

Here's a sample demonstration:

CREATE PROCEDURE procedure1(IN Param1 VARCHAR(255), OUT Param2 VARCHAR(255), OUT Param3 VARCHAR(255))
BEGIN
  SET @c2 = '';
  SET @c3 = '';
  SET @query = 'SELECT column2, column3 INTO @c2, @c3 FROM table1 WHERE column1 = ?';
  PREPARE stmt FROM @query;
  SET @c1 = Param1;
  EXECUTE stmt USING @c1;
  DEALLOCATE PREPARE stmt;
  SET Param2 = @c2;
  SET Param3 = @c3;
END$$

Calling the procedure and using the variables:

SET @Param1 = 2;
SET @Param2 = '';
SET @Param3 = '';
CALL procedure1(@Param1, @Param2, @Param3);
SELECT @Param2, @Param3;

+---------+---------+
| @Param2 | @Param3 |
+---------+---------+
| value3  | value4  |
+---------+---------+

By utilizing this technique, you can execute dynamic queries and retrieve their results into OUT parameters within MySQL stored procedures, enabling you to handle complex data operations efficiently.

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