1

我是 SQL 新手，所以请多多包涵。
我正在写一个登录脚本，想从三个表中获取信息。问题是这个查询只从 table3 返回一个值。
有人可以给我一些指示吗？

SELECT
    table1.id,
    table1.username,
    table1.password,
    table1.email, 
    table2.image,   
    table3.intValue,
    table3.textValue,
    table3.dateValue
FROM
    table1
LEFT JOIN
    table2

ON
    table1.id = table2.userId

LEFT JOIN
    table3
ON
    table1.id = table3.userId
        AND columnName='sex' OR columnName='birthdate' OR columnName='realname'

WHERE 
    table1.email = $username
OR 
    table1.username = $username

columnName='sex' 是一个整数 (intValue)
columnName='birthdate' 是一个日期 (dateValue)
columnName='realname' 是一个字符串 (textValue)

谢谢你。

4

1 回答 1

1

这是您的查询（已格式化，以便我可以更好地阅读）：

SELECT t1.id, t1.username, t1.password, t1.email, 
       t2.image, t3.intValue, t3.textValue, t3.dateValue
FROM table1 t1 LEFT JOIN
     table2 t2
     ON t1.id = t1.userId LEFT JOIN
     table3 t3
     ON t1.id = t3.userId AND
        columnName='sex' OR columnName='birthdate' OR columnName='realname'
WHERE t1.email = $username OR t1.username = $username ;

一个问题是OR. table3这被评估为：

    ON (t1.id = t3.userId AND columnName='sex') OR columnName='birthdate' OR columnName='realname';

SQL 不会读心。它调用优先规则。最好将条件表述为：

     ON t1.id = t3.userId AND
        columnName in ('sex', 'birthdate', 'realname');

但是，我认为这不会导致一排的问题。如果有的话，那将成倍增加行数。

您似乎希望在一行中获取所有值，而您的查询将为table3. 如果是这样，您应该使用group by, 并进行适当的聚合。最终查询将是：

SELECT t1.id, t1.username, t1.password, t1.email, 
       t2.image,
       max(case when columnName = 'sex' then t3.intValue end) as sex,
       max(case when columnName = 'realname' then t3.textValue end) as realname,
       max(case when columnName = 'birthdate' then t3.dateValue end) as birthdate
FROM table1 t1 LEFT JOIN
     table2 t2
     ON t1.id = t1.userId LEFT JOIN
     table3 t3
     ON t1.id = t3.userId AND
        columnName in ('sex', 'birthdate', 'realname')
WHERE t1.email = $username OR t1.username = $username
GROUP BY t1.id;
于 2013-09-07T14:41:40.727 回答