score表
stuid subject score
1 math 80
1 english 90
2 math 81
2 english 91
3 math 85
3 english 95

要求得到的組合查詢結果
id math english sum
1 80 90 170
2 81 91 172
3 85 95 180
avg 82 92 174

(
    SELECT
        stuid as id,
        CAST(sum(case when `subject`
 
='math' then score end) AS SIGNED) as math,
        CAST(sum(case when `subject`='english' then score end) AS SIGNED) as english,
        CAST((
            sum(case when `subject`='math' then score end)
            +
            sum(case when `subject`='english' then score end)
        ) AS SIGNED) as
 
 'sum'
    FROM
        score
    GROUP BY
    stuid
) 
UNION
(
    SELECT
        'avg' as id,
        CAST(avg(case when `subject`='math' then score end) AS SIGNED) as math,
        CAST(avg(case when `subject`='english' then score end) AS SIGNED) as english,
        CAST((
            avg(case when
 
 `subject`='math' then score end)
            +
            avg(case when `subject`='english' then score end)
        ) AS SIGNED) as 'sum'
    FROM
        score
)

union 對兩個結果集進行並集操作，重複資料只顯示一次
union All，對兩個結果集進行並集操作，重複資料全部顯示