Question

我需要一种方法来定义一个变量来保存MYSQL中表达式的结果，这样我就可以在整个查询中使用该变量。
这样就可以了：

SELECT 
IF(VALID,EMPLOYEE_NAME,"NULL") AS NAME,   /** <-- Note *VALID* 
IF(VALID,ID,"NULL") AS ID,   /** <-- And here... 
IF(VALID,OCCUPATION,"NULL") AS OCCUPATION,   /** <-- And here... 

IF((**LONG EXPRESSION**), TRUE, FALSE) AS VALID; /** <-- Taken from here

其中LONG EXPRESSION可能是我需要为IF语句中的每个返回的列放置的非常长的表达式，有很多IF。

我的直觉告诉我，我应该能够像每种程序语言一样：

$var = LONG_EXPRESSION(....);
if($var) {..};
if($var) {..};
if($var) {..};
if($var) {..};

这种方式更合乎逻辑，更不容易出错。易于维护，可读，甚至可能优化。

MYSQL中是否有相应的内容？
那么，为每一行执行定义一个参数？

Answer 1

您只需要一个子查询：

SELECT IF(VALID,EMPLOYEE_NAME,"NULL") AS NAME,   /** <-- Note *VALID* 
       IF(VALID,ID,"NULL") AS ID,   /** <-- And here... 
       IF(VALID,OCCUPATION,"NULL") AS OCCUPATION,
       ...,
       VALID
from (select t.*, IF((**LONG EXPRESSION**), TRUE, FALSE) AS VALID
      from t
    ) t

Answer 2

执行此操作的更合适的方法可能是在原始表和带有包含long表达式的OUTER JOIN子句的子查询之间执行WHERE。

SELECT IF(b.rowid IS NULL, "NULL", a.employee_name) AS name,
       IF(b.rowid IS NULL, "NULL", a.id) AS id,
       IF(b.rowid IS NULL, "NULL", a.occupation) AS occupation,
       ...,
       IF(b.rowid IS NULL, 0, 1) AS valid
FROM mytable a
LEFT OUTER JOIN
(SELECT rowid FROM mytable
 WHERE **LONG EXPRESSION**) b
ON a.rowid = b.rowid

这假设rowid是表中的唯一键列。

完成此SQL查询所需的方法

2 个答案: