ALTER TABLE脚本中的MySQL变量 [英] MySQL variables in ALTER TABLE script

问题描述

Hello以下过程将必须将所有约束从一个表移动到另一个表，但是我在约束应该删除的时候遇到一些困难。

Hello The following procedure will have to move all constraints from one table to the other however I am having some difficulties at the point where the constraint should be deleted.

问题：如何在下面行中使用变量

The problem: how do I use variables in the following line

ALTER TABLE var_referenced_table_name DROP FOREIGN KEY var_constraint_name;

当我使用时，我收到以下错误

when I use as is, I receive the following error

Error Code: 1146. Table 'oaf_businesslink_dev.var_referenced_table_name' doesn't exist

MySQL无法识别 var_referenced_table_name 和 var_constraint_name 作为变量。

MySQL does not recognise var_referenced_table_name and var_constraint_name as variables.

DELIMITER //
DROP PROCEDURE IF EXISTS AlterConstraints//
CREATE PROCEDURE AlterConstraints()
BEGIN

    DECLARE schema_name VARCHAR(60) DEFAULT 'oaf_businesslink_dev';
    DECLARE table_name VARCHAR(60) DEFAULT 'wp_systemuser';

    DECLARE finished INTEGER DEFAULT 0;
    DECLARE total INTEGER DEFAULT 0;    

    DECLARE var_constraint_name VARCHAR(60) DEFAULT '';
    DECLARE var_table_name VARCHAR(60) DEFAULT '';
    DECLARE var_column_name VARCHAR(60) DEFAULT '';
    DECLARE var_referenced_table_name VARCHAR(60) DEFAULT '';
    DECLARE var_referenced_column_name VARCHAR(60) DEFAULT '';

    DECLARE cur_constraints CURSOR FOR SELECT constraint_Name, table_name,column_name,referenced_table_name,referenced_column_name
    FROM information_schema.key_column_usage
    WHERE constraint_schema = schema_name
        AND referenced_table_name = table_name
        AND table_name IS NOT NULL;

    DECLARE CONTINUE HANDLER FOR NOT FOUND SET finished = 1;
    OPEN cur_constraints;

    get_constraint: 
    LOOP FETCH cur_constraints 
    INTO var_constraint_name
        ,var_table_name
        ,var_column_name
        ,var_referenced_table_name
        ,var_referenced_column_name;          

        IF finished THEN
            LEAVE get_constraint;
        END IF; 

        /* Get Constraint Count */
        SET total = total + 1;

        /* Remove Constraint */
        IF EXISTS(SELECT * FROM information_schema.TABLE_CONSTRAINTS
            WHERE CONSTRAINT_NAME = var_constraint_name AND TABLE_NAME = var_referenced_table_name AND TABLE_SCHEMA = schema_name)
            THEN
            /* 
             * Error Code: 1146. Table 'oaf_businesslink_dev.var_referenced_table_name' doesn't exist
             */
            ALTER TABLE var_referenced_table_name DROP FOREIGN KEY var_constraint_name;
        END IF;

        /* Change Datatype to BIGINT */

        /* Recreate Constraint to new table */
    END

    LOOP get_constraint;
    CLOSE cur_constraints;
    SELECT total;

END
//
DELIMITER ;

CALL AlterConstraints();

提前感谢。

推荐答案

使用变量作为列名和表，最好将 DECLARE 作为字符串，然后执行该字符串通过 准备的语句 。

With the use of variables as column names and tables, it would be best to DECLARE a query as a "string" and then execute that string via a Prepared Statement.

这可以通过两种方式完成： CONCAT（）构建完整的字符串或使用 PREPARE 和参数：

This can be done in two ways, either by CONCAT() to build the full string or by using PREPARE with arguments:

SET @query = CONCAT('ALTER TABLE ', var_referenced_table_name, ' DROP FOREIGN KEY ', var_constraint_name, ';');
PREPARE stmt FROM @query; 
EXECUTE stmt; 
DEALLOCATE PREPARE stmt;

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