php - 对多个表进行 COUNT(*)

Question

我需要统计有多少用户在线。我把它放在两个表中，这是第一个表:

 +-------------+-------------+-------------+
 |      id     |     name    |    status   |
 +-------------+-------------+-------------+
 |      1      |     name    |    1        |
 +-------------+-------------+-------------+
 |      2      |     name    |    1        |
 +-------------+-------------+-------------+
 |      3      |     name    |    0        |
 +-------------+-------------+-------------+

第二个表:

 +-------------+-------------+-------------+
 |      id     |    userid   |  friend_id  |
 +-------------+-------------+-------------+
 |      1      |      1      |    2        |
 +-------------+-------------+-------------+
 |      2      |      1      |    2        |
 +-------------+-------------+-------------+
 |      3      |      2      |    3        |
 +-------------+-------------+-------------+

即 user_id = 1 的另一张 table 上有多少用户以及有多少用户在线。

Answer 1

您可以将表格连接在一起，然后进行计数。

SELECT COUNT(*)
FROM table1 AS t1 
JOIN table2 AS t2 ON (t1.id = t2.friendID)
WHERE t2.userID = 1