如何使用嵌套别名使查询更简单？

Posted 2023-05-09

【中文标题】如何使用嵌套别名使查询更简单？

【英文标题】：How to use nested alias to make the query simpler?

【发布时间】：2018-04-22 17:24:47

【问题描述】：

假设我有这个查询：`

SELECT
    IF(uPer="Yes", "Y", "N") AS Manage,
    IF(date_check="Yes", "Y", "N") As dCheck,
    IF(dCheck="Yes", "Great", "Not Great") AS firstNested,
    IF(firstNested="Great",1,0) AS secondNested
FROM table

所以在这个例子中，我有不止 1 个别名，我想在其他别名中使用其中的一些，就像嵌套别名一样。

如何在不复制/粘贴别名的情况下实现这一点，例如：

IF(
   IF(dCheck="Yes", "Great", "Not Great") AS firstNested) ="Great",1,0
) AS secondNested

注意上面的语法是否正确。

【问题讨论】：

你需要使用子查询。

【参考方案1】：

试试这个：

SELECT
    Manage,
    dCheck,
    firstNested,
    IF(firstNested="Great",1,0) AS secondNested
FROM
(
    SELECT
        Manage,
        dCheck,
        IF(dCheck="Yes", "Great", "Not Great") AS firstNested
    FROM
    (
        SELECT
            IF(uPer="Yes", "Y", "N") AS Manage,
            IF(date_check="Yes", "Y", "N") As dCheck
            FROM table
    ) as t1
) as t2

错误：dCheck 结果将是 Y 或 N，而您在下一个测试中检查 Yes，这将导致错误，firstNested 将是 Not Great 对于所有行。

您的查询应该是：

SELECT
    Manage,
    dCheck,
    firstNested,
    IF(firstNested="Great",1,0) AS secondNested
FROM
(
    SELECT
        Manage,
        dCheck,
        IF(dCheck="Y", "Great", "Not Great") AS firstNested
    FROM
    (
        SELECT
            IF(uPer="Yes", "Y", "N") AS Manage,
            IF(date_check="Yes", "Y", "N") As dCheck
            FROM table
    ) as t1
) as t2