Question

跟随SELECTing the contents of the Table, which is a result of another value，我想在生成的字段上保留一个条件。如果我执行此查询：

SELECT *, (
    SELECT `TableName` FROM `TableNames` WHERE `TableID`=`IndexType`
) AS `IndexTypeName`, 
CASE WHEN `IndexType`=1 THEN (
        SELECT `Username` FROM `Users` WHERE `IndexRowID`=`UserID`
   ) WHEN `IndexType`=2 THEN (
        SELECT `MessageContent` FROM `Messages` WHERE `IndexRowID`=`MessageID`
   ) WHEN `IndexType`=3 THEN (
        SELECT `CommentContent` FROM `Comments` WHERE `IndexRowID`=`CommentID`
   )
END `TableValue`
FROM `Index`
ORDER BY `IndexTime` DESC;

我显然会得到：

+---------+-----------+------------+---------------------+------------+---------------------+
| IndexID | IndexType | IndexRowID | IndexTime           | IndexTable | TableValue          |
+---------+-----------+------------+---------------------+------------+---------------------+
| 5       | 2         | 2          | 2015-04-10 11:45:00 | Messages   | Hello, Mr. Prave... |
+---------+-----------+------------+---------------------+------------+---------------------+
| 4       | 3         | 1          | 2015-04-10 11:30:00 | Comments   | @You, you aren't... |
+---------+-----------+------------+---------------------+------------+---------------------+
| 3       | 2         | 1          | 2015-04-10 11:25:00 | Messages   | Thanks for your ... |
+---------+-----------+------------+---------------------+------------+---------------------+
| 2       | 1         | 2          | 2015-04-10 10:55:00 | Users      | Jeff Atwood         |
+---------+-----------+------------+---------------------+------------+---------------------+
| 1       | 1         | 1          | 2015-04-10 10:50:00 | Users      | Praveen Kumar       |
+---------+-----------+------------+---------------------+------------+---------------------+

但是，如果我想过滤TableValue的值，那么如果我提供的查询如下：

SELECT *, (
    SELECT `TableName` FROM `TableNames` WHERE `TableID`=`IndexType`
) AS `IndexTypeName`, 
CASE WHEN `IndexType`=1 THEN (
        SELECT `Username` FROM `Users` WHERE `IndexRowID`=`UserID`
   ) WHEN `IndexType`=2 THEN (
        SELECT `MessageContent` FROM `Messages` WHERE `IndexRowID`=`MessageID`
   ) WHEN `IndexType`=3 THEN (
        SELECT `CommentContent` FROM `Comments` WHERE `IndexRowID`=`CommentID`
   )
END `TableValue`
FROM `Index`
WHERE `TableValue` LIKE '%prav%'
ORDER BY `IndexTime` DESC;

我收到以下错误：

查询错误（1054）：'where子句'中的未知列'TableValue'。

我不想在子SELECT查询中的每个案例中使用过滤器。是否有其他方法可以通过不执行以下操作：

在每个子WHERE语句中使用SELECT。
为整个事物创建视图。

提前致谢。

Answer 1

您无法在fatal: 'origin' does not appear to be a git repository fatal: Could not read from remote repository. Please make sure you have the correct access rights and the repository exists.中使用别名您需要使用where clause或subquery

having clause

或

SELECT *, (
    SELECT `TableName` FROM `TableNames` WHERE `TableID`=`IndexType`
) AS `IndexTypeName`, 
CASE WHEN `IndexType`=1 THEN (
        SELECT `Username` FROM `Users` WHERE `IndexRowID`=`UserID`
   ) WHEN `IndexType`=2 THEN (
        SELECT `MessageContent` FROM `Messages` WHERE `IndexRowID`=`MessageID`
   ) WHEN `IndexType`=3 THEN (
        SELECT `CommentContent` FROM `Comments` WHERE `IndexRowID`=`CommentID`
   )
END `TableValue`
FROM `Index`
having `TableValue` LIKE '%prav%'
ORDER BY `IndexTime` DESC;

Answer 2

您可以使用HAVING子句来使用别名。

试试这个

SELECT *, (
    SELECT `TableName` FROM `TableNames` WHERE `TableID`=`IndexType`
) AS `IndexTypeName`, 
CASE WHEN `IndexType`=1 THEN (
        SELECT `Username` FROM `Users` WHERE `IndexRowID`=`UserID`
   ) WHEN `IndexType`=2 THEN (
        SELECT `MessageContent` FROM `Messages` WHERE `IndexRowID`=`MessageID`
   ) WHEN `IndexType`=3 THEN (
        SELECT `CommentContent` FROM `Comments` WHERE `IndexRowID`=`CommentID`
   )
END `TableValue`
FROM `Index`
HAVING `TableValue` LIKE '%prav%'
ORDER BY `IndexTime` DESC;

Answer 3

您可以在SQL中使用with子句，您可以在查询的输出中应用过滤器。它在SQL中使用with调用公共类型表达式。这更有效率。

查询错误（1054）：'where子句'中的未知列'TableValue'

3 个答案: