Question

CREATE PROCEDURE data1 (IN v_dt2 VARCHAR(256) , v_col2 VARCHAR(256),col2 VARCHAR(256))
BEGIN

 SELECT dt1 as datep, var1 as vars, 'new' as type 
 FROM samp 
 UNION all
 SELECT v_dt2, v_col2, 'col2' as type 
 FROM samp 
 ;

END //
DELIMITER ;

CALL data1 ("dt2","var2","old")

在上面的括号中的调用语句是列名但是在输出中将参数作为文本而不考虑作为列名...如何在UNION ALL之后的select语句中将它们转换为列名

Answer 1

您的过程返回常量值，而不是列名。要做你想做的事，你需要使用动态SQL（prepare和exec）：

CREATE PROCEDURE data1 (IN v_dt2 VARCHAR(256), v_col2 VARCHAR(256), col2 VARCHAR(256))
BEGIN
    set @s = '
SELECT dt1 as datep, var1 as vars, ''new'' as type 
FROM samp 
UNION all
SELECT @v_dt2, @v_col2, ''col2'' as type 
FROM samp ';

    set @s = replace(@s, '@v_dt2', v_dt2);
    set @s = replace(@s, '@v_col2', v_col2);
    set @s = replace(@s, '@col2', col2);

    prepare s from @s;
    exec s;

    deallocate prepare s;
END //
DELIMITER ;

将列名称作为参数传递并调用

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