Question

我正在创建一个MySQL存储过程，该存储过程将接收两个视图的名称，并对它们执行一个Union合并为两个视图中的第一个。

我已经使用准备好的语句将视图名称作为字符串，所以我可以将此存储过程与其他许多产生不同名称的视图的存储过程一起使用。

以下代码按预期工作：

DROP view if exists test1;
DROP view if exists test2;

CREATE VIEW test1 as SELECT "Cows";
CREATE VIEW test2 as SELECT "Horses";

DROP VIEW if exists tempView;
CREATE VIEW tempView AS SELECT * FROM test1 UNION SELECT * FROM test2;
SELECT * from tempView;

但是，当我执行以下代码时：

DELIMITER //
DROP PROCEDURE IF EXISTS SP_unionViews //
CREATE PROCEDURE SP_unionViews(IN viewname varchar(255), 
                               IN viewname2 varchar(255))
BEGIN
    DROP VIEW IF EXISTS tempView;
    SET @in1 = viewname;
    SET @in2 = viewname2;
    SET @str = 'CREATE VIEW tempView AS 
       SELECT * FROM ? UNION SELECT * FROM ?';
    PREPARE stmt FROM @str;
    EXECUTE stmt USING @in1, @in2;

    SET @str2 = 'DROP VIEW ?';
    SET @in3 = viewname;
    PREPARE stmt2 FROM @str2;
    EXECUTE stmt2 USING @in3;

    SET @str3 = 'CREATE VIEW ? AS SELECT * FROM tempView';
    PREPARE stmt3 FROM @str3;
    EXECUTE stmt3 USING @in3;

    DEALLOCATE PREPARE stmt;
    DEALLOCATE PREPARE stmt2;
    DEALLOCATE PREPARE stmt3;
END //
DELIMITER ;

DROP view if exists test1;
DROP view if exists test2;

CREATE VIEW test1 as SELECT "Cows";
CREATE VIEW test2 as SELECT "Horses";
CALL SP_unionViews(test1, test2);
SELECT * from test1;

我收到以下错误：

#1054 - Unknown column 'test1' in 'field list'

这似乎表明此存储过程正在尝试将test1用作一列，而不是原来打算使用的列。但是我不知道在哪里。

更新：当按如下方式进行编辑时，我希望该解决方案能够正常工作，但是它给了我另一个错误。

新代码为：

DELIMITER //
DROP PROCEDURE IF EXISTS SP_unionViews //
CREATE PROCEDURE SP_unionViews(IN viewname varchar(255), 
                               IN viewname2 varchar(255))
BEGIN
    DROP VIEW IF EXISTS tempView;
    SET @str = CONCAT('
    CREATE VIEW tempView AS 
        SELECT * FROM ', viewname, ' UNION SELECT * FROM ', viewname2);
    PREPARE stmt FROM @str;
    EXECUTE stmt;
    DEALLOCATE PREPARE stmt;

    SET @str2 = CONCAT('DROP VIEW ', viewname, ';');
    PREPARE stmt2 FROM @str2;
    EXECUTE stmt2;
    DEALLOCATE PREPARE stmt2;

    SET @str3 = CONCAT('CREATE VIEW ', viewname ,' AS SELECT * FROM tempView');
    PREPARE stmt3 FROM @str3;
    EXECUTE stmt3;
    DEALLOCATE PREPARE stmt3;
END //
DELIMITER ;

DROP view if exists test1;
DROP view if exists test2;

CREATE VIEW test1 as SELECT "Cows";
CREATE VIEW test2 as SELECT "Horses";
CALL SP_unionViews("test1", "test2");
SELECT * from test1;

其错误显示为：

#1615 - Prepared statement needs to be re-prepared

Answer 1

在准备好的语句中不能对表名使用参数，因此在准备语句之前，需要将表名放入查询字符串中。尝试将查询更改为以下内容：

SET @str = CONCAT('
CREATE VIEW tempView AS 
    SELECT * FROM (
        SELECT * FROM ', viewname, ' UNION SELECT * FROM ', viewname2, '
    )');

SET @str2 = CONCAT('DROP VIEW ', viewname);

SET @str3 = CONCAT('CREATE VIEW ', viewname, ' AS SELECT * FROM tempView');

完成此操作后，您将不再需要EXECUTE的任何参数

在MySQL存储过程中合并视图

1 个答案: