mysql count 统计多表数据

mysql 统计多表

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select * from ((select count(*) as countnum from a_admin_code where classid=6) a,(select count(*) as countnum from a_admin_code_class where fid=1) b,(select count(*) as countnum from a_admin_company)c )


countnum   countnum   countnum
3		5		1


select a.countnum+b.countnum+c.countnum as allnum from ((select count(*) as countnum from a_admin_code where classid=6) a,(select count(*) as countnum from a_admin_code_class where fid=1) b,(select count(*) as countnum from a_admin_company)c )

allnum
9

统计一张表-------------------------------------------------------------------------------------------

SELECT a.classid,count(*) as tjnum,b.classname,b.id FROM `a_admin_code` as a left join a_admin_code_class as b on a.classid=b.id group by classid
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union all统计多表
现在需要把它们汇总加起来，可以使用如下方法实现
SELECT COUNT(cnt) FROM (
SELECT COUNT(id) AS cnt FROM test_0 WHERE field1 = 1
UNION ALL
SELECT COUNT(id) AS cnt FROM test_1 WHERE field1 = 1
.
.
.
UNION ALL
SELECT COUNT(id) AS cnt FROM test9 WHERE field1 = 1
) AS countdata

注：为UNION ALL的所有表做一个别名是必须的，虽然这个别名并没有什么意义
但如果没有的话mysql会报出错误：Every derived table must have its own alias
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mysql count多表数据总和
SELECT a.countNum + b.countNum + c.countNum
FROM (
(SELECT COUNT(*) AS countNum FROM table_a) a,
(SELECT COUNT(*) AS countNum FROM table_b) b,
(SELECT COUNT(*) AS countNum FROM table_c) c
)