在 MySQL 存储过程中合并视图

Question

我正在创建一个 MySQL 存储过程，它接收两个视图的名称，并在它们上执行联合到两个视图中的第一个。

我使用准备好的语句将视图名称作为字符串，因此我可以将此存储过程与大量其他存储过程一起使用，这些存储过程会生成不同名称的视图。

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 作为列使用它不应该是一个的地方。但我不知道在哪里。

更新： 如下编辑时，我希望解决方案能够工作，但它给了我另一个错误。

新代码如下：

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;

它的错误是：

#1615 - Prepared statement needs to be re-prepared

Answer 1

您不能在准备好的语句中使用表名参数，因此您需要在准备语句之前将表名放入查询字符串中。尝试将您的查询更改为：

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 EXECUTEs 的任何参数