Mysql 如何合计3个select语句计数？

我有三个选择声明如下，我想总结一下记录的总数。我该怎么做

SELECT COUNT(*) AS Number FROM tableA where user_id = 5   //Total 5 records
SELECT COUNT(*) AS Number FROM tableB where user_id = 5   //Total 6 records
SELECT COUNT(*) AS Number FROM tableC where user_id = 5   //Total 1 records

---

因此，返回的结果将是12。

我认为这应该可以做到：

SELECT SUM(Number)
FROM (
    SELECT COUNT(*) AS Number FROM tableA where user_id = 5
    UNION ALL
    SELECT COUNT(*) AS Number FROM tableB where user_id = 5
    UNION ALL
    SELECT COUNT(*) AS Number FROM tableC where user_id = 5
)

---

您可以将

count（*）

应用于

union all

的结果：

SELECT COUNT(*)
FROM   (SELECT user_id FROM tablea
        UNION ALL
        SELECT user_id FROM tableb
        UNION ALL
        SELECT user_id FROM tablec) t
WHERE  user_id = 5

SELECT COUNT(*)
FROM   (SELECT user_id FROM tablea
        UNION ALL
        SELECT user_id FROM tableb
        UNION ALL
        SELECT user_id FROM tablec) t
WHERE  user_id = 5