How to Capture Output of Dynamic Queries into OUT Parameters in MySQL Stored Procedures?

Outputting Query Results to Variables in Stored Procedures with Dynamic Queries

Dynamic queries offer flexibility in database programming. In MySQL, it's possible to generate dynamic queries within stored procedures and execute them to obtain results. However, a common question arises: how to capture the output of such queries into OUT parameters in a stored procedure?

Consider the following example where a dynamic query is constructed to count records (query1) and retrieve data (query2):

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

 SET query1 = 'SELECT COUNT(*) bla bla bla.....'; 
 // Query1 to select the count of matching tuples..

 SET query2 = 'SELECT * from bla bla bla....';
 // Query2 to select original records...

 // later part of this both queries generate dynamically according to some IN parameters..

 // now I wanna assign the output of the query1 into numOfRecords 
 // and I wanna execute the query2 as well.. like this

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

 // output of the query2 can be read in PHP

END

To resolve this, we can use a modified approach as illustrated below:

CREATE TABLE table1(
  column1 VARCHAR(255) DEFAULT NULL,
  column2 VARCHAR(255) DEFAULT NULL,
  column3 VARCHAR(255) DEFAULT NULL
);

INSERT INTO table1 VALUES 
  ('1', 'value1', 'value2'),
  ('2', 'value3', 'value4');

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

-- Call procedure and use variables
SET @Param1 = 2;
SET @Param2 = '';
SET @Param3 = '';
CALL procedure1(@Param1, @Param2, @Param3);
SELECT @Param2, @Param3;

This method initializes variables (@c2 and @c3) to hold the output of the dynamic query and constructs the query (@query) to populate these variables with data from the database. The query is then prepared, executed with the specified parameter, and the results are assigned to the OUT parameters (Param2 and Param3) before exiting the procedure.

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