Question

您好我有以下树表，我想实现名为“想要的结果”的表。第一个是房子名称，第二个是每个房间的房间名称，第三个是每个房间的价格。我想要一个房间的房间数。

TABLE_1

nid    name
1      House 1
2      House 2 
3      House 3

TABLE_2

Name     house_ref_id    id
Room1    1               20
Room1    2               21
Room1    2               22
Room1    3               23
Room1    3               24

TABLE_3

Price    room_ref_id
0        20
100      21
150      22
0        23
120      24

通缉结果：

nid      name       Rooms w/ price
1        House1     0
2        House2     2
3        House3     1

我尝试了以下代码，但可以获得价格合适的房间

SELECT Name, COUNT(price) AS "rooms w/ price" FROM table_1
LEFT JOIN table_2 ON table_1.id = table_2.ref_id
LEFT JOIN table_3 ON table_2.room_ref_id = table_3.id 
WHERE table_3.price>0
GROUP BY table_1.nid

但得到以下内容：

nid     name      rooms w/ price
1       nouse1    1
2       house2    2
3       house3    2

Answer 1

关闭。 WHERE中的条件将外连接转换为内连接。您需要将其移至ON子句：

SELECT Name, COUNT(table3.id) AS "rooms w/ price"
FROM table_1 LEFT JOIN
     table_2 
     ON table_1.id = table_2.ref_id LEFT JOIN
     table_3
     ON table_2.room_ref_id = table_3.id AND table_3.price > 0
GROUP BY table_1.nid;

SQL-从表中获取计数（LEFT JOIN）和条件

1 个答案: