How to get number of specific rows from a different table in a subquery

问题

I know it's possible, but I'm not experienced enough to know how to do subqueries.
Here's the situation:

Table 1:
+--------------------+--------------------+
|               v_id |             v_name |
+--------------------+--------------------+
|                  1 |            v_name1 |
+--------------------+--------------------+
| etc...

Table 2:
+--------------------+--------------------+
|               a_id |             a_name |
+--------------------+--------------------+
|                  1 |            a_name1 |
+--------------------+--------------------+
| etc...

Table 3:
+--------------------+--------------------+
|               v_id |               a_id |
+--------------------+--------------------+
|                  1 |                  1 |
+--------------------+--------------------+
|                  1 |                  2 |
+--------------------+--------------------+
|                  1 |                  3 |
+--------------------+--------------------+
|                  2 |                  3 |
+--------------------+--------------------+
|                  2 |                  1 |
+--------------------+--------------------+

I believe this is a quite common situation.
So, I have unique entries in Table 1 and Table 2.
I want to SELECT all rows from Table 1 and get (as the last cell in each row) the number of rows with the corresponding value in Table 3.

This doesn't work:

SELECT t1.* , COUNT(SELECT t3.* FROM  `table_3` t3 WHERE t3.v_id = t1.v_id) as entries
FROM  `table 1` t1;

I'm sure I'm gonna be told off by experts here that it's all wrong, but frankly, that's what I'm looking for (and some helpful solution as well!). ;)

回答1:

Use:

   SELECT t1.*,
          COALESCE(x.num_rows, 0) AS entries
     FROM `table 1` t1
LEFT JOIN (SELECT t3.v_id,
                  COUNT(*) 'num_rows'
             FROM `table_3` t3
         GROUP BY t3.v_id) x ON x.v_id = t1.v_id

回答2:

SELECT t1.* , (SELECT COUNT(*) FROM  `table_3` t3 WHERE t3.v_id = t1.v_id) as t3Count as entries 
FROM  `table 1` t1;

回答3:

SELECT T1.v_id, COALESCE(COUNT(T3.v_id), 0)
FROM Table1 AS T1
LEFT JOIN Table3 AS T3
ON T1.v_id = T3.v_id
GROUP BY T1.v_id

来源：https://stackoverflow.com/questions/2143116/how-to-get-number-of-specific-rows-from-a-different-table-in-a-subquery

标签

sql

mysql

subquery