mysql - How to evaluate a formula in varchar column of MYSQL Table

Question

Refer to the below SQL. We need to save STD formula in table, but not able to evaluate this.

    DROP TEMPORARY TABLE IF EXISTS _std ;
CREATE TEMPORARY TABLE _std(stdformula VARCHAR(4000), mu DECIMAL(18,9), newValue DECIMAL(18,9));

INSERT INTO _std VALUES('SQRT((POWER((5-mu), 2) + POWER((6-mu), 2)  + POWER((7-mu), 2) + POWER((7-mu), 2) )/4)',5+6+7+7/4,7);

SET @temp=(SELECT REPLACE(stdformula,'mu',mu) FROM _std );
SELECT @temp;
-- @temp=> SQRT(( POWER((5-19.750000000), 2) + POWER((6-19.750000000), 2)  + POWER((7-19.750000000), 2) + POWER((7-19.750000000), 2) )/2);
-- which is valid statement
SELECT SQRT(( POWER((5-19.750000000), 2) + POWER((6-19.750000000), 2)  + POWER((7-19.750000000), 2) + POWER((7-19.750000000), 2) )/2);

-- How can we evaluate @temp using dynamic sql approach?? -- Below prepare is not working

PREPARE StmtStd FROM @temp;
EXECUTE StmtStd; 
DEALLOCATE PREPARE Stmtstd;*

Answer 1

你SELECT在@temp. 尝试

...
SET @temp= CONCAT('SELECT ', (SELECT REPLACE(stdformula,'mu',mu) FROM _std ), ' result');
-- SELECT @temp;    
PREPARE StmtStd FROM @temp;
EXECUTE StmtStd;

输出：

|          RESULT |
-------------------
| 13.525438994724 |

这是SQLFiddle演示