3个表的MySQL连接-从2个子表中获取合计值

Question

I need to create a summary from 3 different tables, 1 parent table, 2 child tables.

How can I get the number of records from two child tables, based on the user id (pk in each of the 3 tables).

Parent table ( user ) pk is userId Child tables 1 and 2 have composite pks of userId and webId.

I know this isn't the proper SQL syntax, but it illustrates what I'm after.

select u.userId, count(table1.webId), count(table2.webId)
from `user` u 
left join `table1` t1 on u.userId = t1.userId
left join `table2` t2 on u.userId = t2.userId
group by u.userId

Answer 1

You might need to add DISTINCT -

SELECT u.userId, COUNT(DISTINCT table1.webId), COUNT(DISTINCT table2.webId)
FROM `user` u 
LEFT JOIN `table1` t1
    ON u.userId = t1.userId
LEFT JOIN `table2` t2
    ON u.userId = t2.userId
GROUP BY u.userId