需求
产品表：product
标签表：tag
中间表：tagassoc
关联关系：product.id = tagassoc.objectID,
tag.id = tagassoc.tagid
一个产品可以有多个标签, 标签删除后, tagassoc.tagid值为0
查询产品中有标签的数量和没有标签的数量
实现参考
SELECT IF (t.id IS NULL, -1, t.id) AS id, count(p.id) AS count FROM product AS p LEFT JOIN `tagassoc` AS tc ON tc.objectType = 2 AND tc.objectID = p.id AND tc.tagid != 0 LEFT JOIN `tag` AS t ON t.id = tc.tagid WHERE p.deleted = '0' AND CASE WHEN t.deleted IS NULL THEN '0' ELSE t.deleted END = '0' AND p. STATUS = '0' GROUP BY t.id
结果
| id | count |
| -1 | 13045 |
| 1 | 2 |
| 2 | 3 |
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