Question

这似乎可行，但告诉显示其值不为零的记录，所有似乎都告诉它不显示值为NULL的记录。（即不在tableB中的记录）。在mysql中，似乎0等于NULL。如何获得我正在寻找的结果？

SELECT DISTINCT Name, CategoryID
FROM sockcategory 
LEFT JOIN sockcategoryparentxref ON 
sockcategory.CategoryID = sockcategoryparentxref.Category_ID 
WHERE CategoryParentID !=0;

架构/样本数据

表sockcategory：

+-----------+------------+
|   Name    | CategoryID |
+-----------+------------+
| Red       |          1 |
| Blue      |          2 |
| Green     |          3 |
| Colors    |          4 |
| Wholesale |          5 |
+-----------+------------+

表sockcategoryparentxref：

+-------------+------------------+
| Category_ID | CategoryParentID |
+-------------+------------------+
|           1 |                4 |
|           2 |                4 |
|           3 |                4 |
|           4 |                0 |
+-------------+------------------+

期望的输出：

+-----------+------------+
|   Name    | CategoryID |
+-----------+------------+
| Red       |          1 |
| Blue      |          2 |
| Green     |          3 |
| Wholesale |          5 |
+-----------+------------+

Answer 1

如果我理解正确，这就是你要找的东西：

SELECT DISTINCT `Name`, `CategoryID`
FROM `sockcategory` 
LEFT JOIN `sockcategoryparentxref`
    ON `sockcategory`.`CategoryID` = `sockcategoryparentxref`.`Category_ID`
    AND `CategoryParentID` <> 0;

Answer 2

我认为您的意思是比较WHERE CategoryParentID IS NULL，而不是假设CategoryParentID属于sockcategoryparentxref表。

select DISTINCT s.Name,
s.CategoryID 
FROM sockcategory s
LEFT JOIN sockcategoryparentxref so ON 
s.CategoryID = so.Category_ID 
WHERE so.CategoryParentID IS NULL;

如果没有，则考虑将条件移至JOIN ON子句

select DISTINCT s.Name,
s.CategoryID 
FROM sockcategory s
LEFT JOIN sockcategoryparentxref so ON 
s.CategoryID = so.Category_ID 
AND so.CategoryParentID <> 0;

MySQL过滤加入

2 个答案: