SQL Fundamentals SQL列字段的处理 SQL（case、decode、行转列）

列字段的处理 SQL

Case

对列做处理

SQL> select deptno,sum(sal) from emp group by deptno;

Select

case

when deptno=10 then ‘ACCOUNTING‘

when deptno=20 then ‘RESERCH‘

when deptno=30 then ‘SALES‘

end,

sum(sal) from emp

group by deptno

Decode

还有一种情况是当什么都不是的情况下，用一个值代替

前言

DECODE()函数，它将输入数值与函数中的参数列表相比较，根据输入值返回一个对应值。函数的参数列表是由若干数值及其对应结果值组成的若干序偶形式。当然，如果未能与任何一个实参序偶匹配成功，则函数也有默认的返回值。

语法结构如下：

decode (expression, search_1, result_1)
decode (expression, search_1, result_1, search_2, result_2)
decode (expression, search_1, result_1, search_2, result_2, ...., search_n, result_n)

decode (expression, search_1, result_1, default)
decode (expression, search_1, result_1, search_2, result_2, default)
decode (expression, search_1, result_1, search_2, result_2, ...., search_n, result_n, default)

比较表达式和搜索字，如果匹配，返回结果；如果不匹配，返回default值；如果未定义default值，则返回空值。

select

decode(deptno,

10, ‘ACCOUNTING‘,

20, ‘RESERCH‘,

30, ‘SALES‘

),

sum(sal) from emp

group by deptno

SQL> select job,ename,sal from emp where job=‘MANAGER‘;

JOB       ENAME             SAL

--------- ---------- ----------

MANAGER   JONES            2975

MANAGER   BLAKE            2850

MANAGER   CLARK            2450

SQL>

SQL> select job,decode(ename,‘BLAKE‘,SAL) BLAKE,decode(ename,‘JONES‘,SAL) JONES,decode(ename,‘CLARK‘,SAL) CLARK from emp where job=‘MANAGER‘;

JOB            BLAKE      JONES      CLARK

--------- ---------- ---------- ----------

MANAGER                    2975

MANAGER         2850

MANAGER                               2450

SQL> select job,sum(decode(ename,‘BLAKE‘,SAL)) BLAKE,sum(decode(ename,‘JONES‘,SAL)) JONES,sum(decode(ename,‘CLARK‘,SAL)) CLARK from emp where job=‘MANAGER‘ group by job;

JOB            BLAKE      JONES      CLARK

--------- ---------- ---------- ----------

MANAGER         2850       2975       2450

SQL> select job,avg(decode(ename,‘BLAKE‘,SAL)) BLAKE,max(decode(ename,‘JONES‘,SAL)) JONES,min(decode(ename,‘CLARK‘,SAL)) CLARK from emp where job=‘MANAGER‘ group by job;

JOB            BLAKE      JONES      CLARK

--------- ---------- ---------- ----------

MANAGER         2850       2975       2450