如果不存在LEFT JOIN，则忽略

Question

并非所有的child行都有parent。该child行有一个parent只应获取如果parent.deleted='0'

如果LEFT JOIN content as parent不存在，我怎么忽略parent.deleted='0'该行？

SELECT child.* FROM `content` child LEFT JOIN 
content parent on parent.id=child.parentid AND child.submittype='2' WHERE child.username=? 
AND child.deleted='0' AND parent.deleted='0' 
ORDER BY child.id DESC LIMIT 12 

我该如何做？我把parent.deleted='0'放在WHERE条款中，但这只有在parent行存在的情况下才有效。

Answer 1

你正在寻找的SQL是

SELECT child.* 
FROM `content` child 
    LEFT JOIN 
     content parent 
on parent.id=child.parentid AND child.submittype='2' 
WHERE child.username=? 
AND child.deleted='0' AND (parent.deleted='0' OR parent.deleted IS NULL) 
ORDER BY child.id DESC LIMIT 12 

Answer 2

一个LEFT JOIN将返回所有的行，即使parentid为NULL。因此，解决您的问题的第一步是使用INNER JOIN。有关说明，请参见this question。

您的结果集现在将只包含行，其中父实际可用。为了进一步过滤这个结果，你的WHERE子句已经存在。不过，为了更好的可读性，我建议在submittype上移动过滤器。总的结果将是：

SELECT child.* FROM `content` child INNER JOIN 
    content parent on parent.id=child.parentid 
WHERE 
    child.username=? AND 
    child.submittype='2'AND 
    child.deleted='0' AND 
    parent.deleted='0' 
ORDER BY child.id DESC LIMIT 12 

Answer 3

我认为你正在寻找的东西是这样的：

$getitem = $connectdb->prepare("SELECT child.* FROM `content` child LEFT JOIN 
content parent ON parent.id=child.parentid AND child.submittype='2' WHERE child.username=? 
AND child.deleted='0' AND (parent.deleted='0' OR parent.deleted IS NULL) 
ORDER BY child.id DESC LIMIT 12"); 

而且你的左边加入只能说目标表加入到这样的：

$getitem = $connectdb->prepare("SELECT child.* FROM `content` child LEFT JOIN 
parent ON parent.id=child.parentid AND child.submittype='2' WHERE child.username=? 
AND child.deleted='0' AND (parent.deleted='0' OR parent.deleted IS NULL) 
ORDER BY child.id DESC LIMIT 12"); 

最后，作为一般规则，我留下了关于连接方向的声明，因此child.submittype也可以移到WHERE子句中：

$getitem = $connectdb->prepare("SELECT child.* FROM `content` child LEFT JOIN 
parent ON parent.id=child.parentid WHERE child.submittype='2' AND child.username=? 
AND child.deleted='0' AND (parent.deleted='0' OR parent.deleted IS NULL) 
ORDER BY child.id DESC LIMIT 12");