Question

我在下面有这个代码。它工作，如果我只使用一个WHERE变量，但当我添加另一个，查询不起作用。

如果我只使用这个所有工会，它就有效：

       where table_constant.user_id = '$uid'

但是当我在下面使用这个时，它不起作用：

      where table_constant.user_id = '$uid' and table_one.something <> '$uid'

代码：

  $sql = "select table_one.field1, table_constant.field1, 
table_one.field2, table_one.field3, table_one.field4, 
table_one.field5, table_constant.c_id
from table_one LEFT JOIN table_constant on table_one.field1 
= table_constant.c_id 
where table_constant.user_id = '$uid' and table_one.something <> '$uid'
UNION
select table_two.field1, table_constant.field1, table_two.field2, 
table_two.field3,    table_two.field4, table_two.field5, table_constant.c_id
from table_two LEFT JOIN table_constant on table_two.c_id 
= table_constant.c_id 
where table_two.added_by = '$uid' and table_two.something <> '$uid'
UNION 
select table_three.field1, table_constant.field1, table_three.field2, 
table_three.field3, table_three.field4, table_three.field5,
table_constant.c_id
from table_three LEFT JOIN table_constant ON table_three.c_id 
= table_constant.c_id
where table_constant.user_id = '$uid' and table_three.something <> '$uid'
UNION
select table_four.field1, table_constant.field1, table_four.field2, 
table_four.field3, table_four.field4, table_four.field5, 
table_constant.c_id
from table_four LEFT JOIN table_constant ON table_four.c_id 
= table_constant.c_id
where table_constant.user_id = '$uid' and table_four.something <> '$uid'
ORDER BY date DESC LIMIT $start, $limit";
$result = mysql_query($sql);

Answer 1

第二个想法我对这个问题可能会有一个很好的猜测。当你写LEFT JOIN时，我认为你这样做是因为可能有一些行与连接右侧的任何内容都不匹配。如果您想以某种方式约束做匹配的行，则需要在JOIN本身中执行此操作。例如：

LEFT JOIN table_constant ON table_one.field1 = table_constant.c_id AND table_constant.user_id = '$uid'

通过将第二个条件放在WHERE子句中，您实际上是通过强制右侧有一个值来将左连接转换为内连接。

如何对UNION有两个约束？

1 个答案: