显示包含子计数的父数据

Question

我有一个数据库tbl_products

id   parent_id  product_name 
1       0           abc
2       1           xyz
3       1           fgh 
4       1           pqr
5       2           lmn
6       2           uvw

我想显示

等数据

id   child_count  product_name 
1       3           abc
2       2           xyz
3       0           fgh 
4       0           pqr
5       0           lmn
6       0           uvw

Answer 1

您可以使用以下查询：

SELECT id,
       (SELECT COUNT(*)
        FROM tbl_products
        WHERE parent_id = t.id) AS child_count,
        product_name
FROM tbl_products AS t

查询使用相关子查询来获取当前记录的子项数。

Demo here

或者，您可以使用JOIN：

SELECT t1.id,
       COALESCE(t2.cnt,0) AS child_count,
       t1.product_name
FROM tbl_products AS t1        
LEFT JOIN (SELECT parent_id, COUNT(*) AS cnt
      FROM tbl_products
      GROUP BY parent_id
) AS t2 ON t1.id = t2.parent_id

Demo here