union?不返回重復行（所有字段值相同的行）

union?all?返回所有行

//?每類最多統計100條

select `server_id`,count(1) as logs from (?SELECT `server_id` FROM `log` WHERE `log`.`type` = "a" AND server_id=1 limit 100?)
UNION
select `server_id`,count(1) as logs from (?SELECT `server_id` FROM `log` WHERE `log`.`type` = "a" AND server_id=2 limit 100?)
UNION
select `server_id`,count(1) as logs from (?SELECT `server_id` FROM `log` WHERE `log`.`type` = "a" AND server_id=3 limit 100?);

SELECT * FROM log WHERE type="a"
UNION
SELECT * FROM log WHERE type="b"
UNION
SELECT * FROM log WHERE type="c"
order by id desc LIMIT 10

【高性能mysql】

(SELECT * FROM log WHERE type="a"?order by id desc LIMIT 10)
UNION?ALL
(SELECT * FROM log WHERE type="b"?order by id desc LIMIT 10)
UNION?ALL
(SELECT * FROM log WHERE type="c"?order by id desc LIMIT 10)
order by id desc LIMIT 10

select sum(num)
from
(
SELECT count(1) as num FROM log WHERE type="a"
UNION
SELECT count(1) as num FROM log WHERE type="b"
UNION
SELECT count(1) as num FROM log WHERE type="c"
)