select sum(case when deptno=10 then 1 else 0 end) as deptno_10,
sum(case when deptno=20 then 1 else 0 end) as deptno_20,
sum(case when deptno=30 then 1 else 0 end) as deptno_30
from emp
1.3.1. 對于每一行的原始資料，使用CASE表達式把行變換成列
1.4. sql
select max(case when deptno=10 then empcount else null end) as deptno_10,
max(case when deptno=20 then empcount else null end) as deptno_20,
max(case when deptno=30 then empcount else null end) as deptno_30
from (
select deptno, count(*) as empcount
from emp
group by deptno
) x
select dept.deptno,
case dept.deptno
when 10 then emp_cnts.deptno_10
when 20 then emp_cnts.deptno_20
when 30 then emp_cnts.deptno_30
end as CNT
from (
select sum(case when deptno=10 then 1 else 0 end) as deptno_10,
sum(case when deptno=20 then 1 else 0 end) as deptno_20,
sum(case when deptno=30 then 1 else 0 end) as deptno_30
from emp
) emp_cnts,
(select deptno from dept where deptno ＜= 30) dept
3. 變換結果內建多行
3.1. 結果集
JOB ENAME
--------- ----------
ANALYST SCOTT
ANALYST FORD
CLERK SMITH
CLERK ADAMS
CLERK MILLER
CLERK JAMES
MANAGER JONES
MANAGER CLARK
MANAGER BLAKE
PRESIDENT KING
SALESMAN ALLEN
SALESMAN MARTIN
SALESMAN TURNER
SALESMAN WARD
3.2. 結果集
CLERKS ANALYSTS MGRS PREZ SALES
------ -------- ----- ---- ------ ---------------
MILLER FORD CLARK KING TURNER
JAMES SCOTT BLAKE MARTIN
ADAMS JONES WARD
SMITH ALLEN
3.3. DB2
3.4. Oracle
3.5. SQL Server
3.6. 使用視窗函數ROW_NUMBER OVER確定每一個JOB/ENAME組合都是唯一的
select max(case when job='CLERK'
then ename else null end) as clerks,
max(case when job='ANALYST'
then ename else null end) as analysts,
max(case when job='MANAGER'
then ename else null end) as mgrs,
max(case when job='PRESIDENT'
then ename else null end) as prez,
max(case when job='SALESMAN'
then ename else null end) as sales
from (
select job,
ename,
row_number()over(partition by job order by ename) rn
from emp
) x
group by rn
3.6.1.1. 為了剔除掉Null，需要調用聚合函數MAX，并基于RN執行GROUP BY
3.7. PostgreSQL
3.8. MySQL
3.9. sql
select max(case when job='CLERK'
then ename else null end) as clerks,
max(case when job='ANALYST'
then ename else null end) as analysts,
max(case when job='MANAGER'
then ename else null end) as mgrs,
max(case when job='PRESIDENT'
then ename else null end) as prez,
max(case when job='SALESMAN'
then ename else null end) as sales
from (
select e.job,
e.ename,
(select count(*) from emp d
where e.job=d.job and e.empno ＜ d.empno) as rnk
from emp e
) x
group by rnk
select case rn
when 1 then ename
when 2 then job
when 3 then cast(sal as char(4))
end emps
from (
select e.ename,e.job,e.sal,
row_number()over(partition by e.empno
order by e.empno) rn
from emp e,
(select *
from emp where job='CLERK') four_rows
where e.deptno=10
) x
5. 删除重複資料
5.1. 結果集
DEPTNO ENAME
------ ---------
10 CLARK
KING
MILLER
20 SMITH
ADAMS
FORD
SCOTT
JONES
30 ALLEN
BLAKE
MARTIN
JAMES
TURNER
WARD
5.1.1. 每個DEPTNO隻顯示一次
5.2. DB2
5.3. SQL Server
5.4. 使用視窗函數MIN OVER
select case when empno=min_empno
then deptno else null
end deptno,
ename
from (
select deptno,
min(empno)over(partition by deptno) min_empno,
empno,
ename
from emp
) x
5.5. Oracle
select to_number(
decode(lag(deptno)over(order by deptno),
deptno,null,deptno)
) deptno, ename
from emp
6. 變換結果集以實作跨行計算
select deptno, sum(sal) as sal
from emp
group by deptno
DEPTNO SAL
------ ----------
10 8750
20 10875
30 9400
select d20_sal - d10_sal as d20_10_diff,
d20_sal - d30_sal as d20_30_diff
from (
select sum(case when deptno=10 then sal end) as d10_sal,
sum(case when deptno=20 then sal end) as d20_sal,
sum(case when deptno=30 then sal end) as d30_sal
from emp
) totals_by_dept
