前言
本篇文章讲解的主要内容是：如何使用lag函数让结果集重复数据只显示一次、用行转列pivot写法优化部门之间计算工资差异类似需求、如何通过ceil函数对已有数据进行分组打印、放假安排团队分组值班，如何通过ntile()over(order by )快速进行人员分组
【SQL开发实战技巧】这一系列博主当作复习旧知识来进行写作，毕竟SQL开发在数据分析场景非常重要且基础，面试也会经常问SQL开发和调优经验，相信当我写完这一系列文章，也能再有所收获，未来面对SQL面试也能游刃有余~。
一、如何让结果集中的重复数据只显示一次
我们返回的数据中经常会有重复值,如EMP.JOB,这些数据经常要求合并显示。这种一般都在前台处理,偶尔也有特殊情况,需要在返回时就只显示第一行数据,该如何处理呢？其实用LAG进行判断即可。
SQL> select job,case when lag(job)over(order by a.job,ename)=job then null else job end as 职位,ename as 姓名
2 from emp a
3 order by a.job,a.ename;
JOB 职位 姓名
--------- --------- ----------
ANALYST ANALYST FORD
ANALYST SCOTT
CLERK CLERK ADAMS
CLERK JAMES
CLERK MILLER
CLERK SMITH
MANAGER MANAGER BLAKE
MANAGER CLARK
MANAGER JONES
PRESIDENT PRESIDENT KING
SALESMAN SALESMAN ALLEN
SALESMAN MARTIN
SALESMAN TURNER
SALESMAN WARD
test
15 rows selected
SQL> 或许有人注意到,order by子句后的job加上了前缀。如果不加前缀,而且列别名仍然是job会出现什么情况？
SQL> select case when lag(job)over(order by a.job,ename)=job then null else job end job,ename
2 from emp a
3 order by job,a.ename;
JOB ENAME
--------- ----------
ANALYST FORD
CLERK ADAMS
MANAGER BLAKE
PRESIDENT KING
SALESMAN ALLEN
CLARK
JAMES
JONES
MARTIN
MILLER
SCOTT
SMITH
TURNER
WARD
test
15 rows selected可以看到,order by子句后优先使用的是"别名",而不是"列名",从而使排序结果与需求不一样。所以大家要养成加"前缀＂的习惯。
二、部门之间计算工资差异时也可以用行转列pivot
基础数据：
SQL> select deptno,sum(sal) as sm from emp where deptno is not null group by deptno ;
DEPTNO SM
------ ----------
10 8750
20 10875
30 9400现在有个需求：要求计算部门20与部门10及部门20与部门30之间的总工资差额。
对于这个需求你会怎么做？是不是像下面这样子？
SQL> select (select sum(sal) from emp where deptno = 20) -
2 (select sum(sal) from emp where deptno = 10) as d20_10,
3 (select sum(sal) from emp where deptno = 20) -
4 (select sum(sal) from emp where deptno = 30) as d20_30
5 from dual;
D20_10 D20_30
---------- ----------
2125 1475
SQL>
Plan Hash Value :
------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost | Time |
------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 14 | 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 7 | | |
| * 2 | TABLE ACCESS FULL | EMP | 5 | 35 | 3 | 00:00:01 |
| 3 | SORT AGGREGATE | | 1 | 7 | | |
| * 4 | TABLE ACCESS FULL | EMP | 3 | 21 | 3 | 00:00:01 |
| 5 | SORT AGGREGATE | | 1 | 7 | | |
| * 6 | TABLE ACCESS FULL | EMP | 5 | 35 | 3 | 00:00:01 |
| 7 | SORT AGGREGATE | | 1 | 7 | | |
| * 8 | TABLE ACCESS FULL | EMP | 6 | 42 | 3 | 00:00:01 |
| 9 | FAST DUAL | | 1 | | 2 | 00:00:01 |
------------------------------------------------------------------------
Predicate Information (identified by operation id):
------------------------------------------
* 2 - filter("DEPTNO"=20)
* 4 - filter("DEPTNO"=10)
* 6 - filter("DEPTNO"=20)
* 8 - filter("DEPTNO"=30)通过执行计划可以看到，我们这个sql扫描了4次emp表，性能挺差啊！那如果让你优化，你该怎么优化呢？
对于这种需求,其实也可以通过"行转列"把各值提到同一行上后,再进行计算。
SQL> with t as
2 （
3 select deptno, sum(sal) as sm
4 from emp
5 where deptno in (10, 20, 30)
6 group by deptno ）
7 select d20_sm - d10_sm, d20_sm - d30_sm
8 from t
9 pivot(max(sm) as sm
10 for deptno in(10 as d10, 20 as d20, 30 as d30));
D20_SM-D10_SM D20_SM-D30_SM
------------- -------------
2125 1475
Plan Hash Value : 3261863285
-------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost | Time |
-------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 39 | 4 | 00:00:01 |
| 1 | VIEW | | 1 | 39 | 4 | 00:00:01 |
| 2 | SORT AGGREGATE | | 1 | 26 | | |
| 3 | VIEW | | 3 | 78 | 4 | 00:00:01 |
| 4 | SORT GROUP BY | | 3 | 21 | 4 | 00:00:01 |
| * 5 | TABLE ACCESS FULL | EMP | 13 | 91 | 3 | 00:00:01 |
-------------------------------------------------------------------------
Predicate Information (identified by operation id):
------------------------------------------
* 5 - filter("DEPTNO"=10 OR "DEPTNO"=20 OR "DEPTNO"=30)可以看到只扫描了一次emp表
三、如何对已有数据进行分组打印
有时为了方便打印,会要求多行多列打印,如emp.ename类似下面这样显示：
ADAMS ALLEN BLAKE CLARK FORD
JAMES JONES KING MARTIN MILLER
SCOTT SMITH TURNER WARD test要达到这个目的,需要以下操作。
- 生成序号
SQL> with t as
2 (select rownum as rn, ename from (select ename from emp order by ename))--第一步，根据名称进行字段顺序排序，排序后取rownum值，因为我想按照字段顺序打印
3 select * from t
4 ;
RN ENAME
---------- ----------
1 ADAMS
2 ALLEN
3 BLAKE
4 CLARK
5 FORD
6 JAMES
7 JONES
8 KING
9 MARTIN
10 MILLER
11 SCOTT
12 SMITH
13 TURNER
14 WARD
15 test
15 rows selected- 通过ceil函数把数据分为几个组
SQL>
SQL> with t as
2 (select rownum as rn, ename from (select ename from emp order by ename)),--第一步，根据名称进行字段顺序排序，排序后取rownum值，因为我想按照字段顺序打印
3 t1 as
4 (select ceil(rn / 5) as gp, ename from t)--我想一页展示五列
5 select * from t
6 ;
RN ENAME
---------- ----------
1 ADAMS
2 ALLEN
3 BLAKE
4 CLARK
5 FORD
6 JAMES
7 JONES
8 KING
9 MARTIN
10 MILLER
11 SCOTT
12 SMITH
13 TURNER
14 WARD
15 test
15 rows selected- 给各组数据生成序号
SQL> with t as
2 (select rownum as rn, ename from (select ename from emp order by ename)),--第一步，根据名称进行字段顺序排序，排序后取rownum值，因为我想按照字段顺序打印
3 t1 as
4 (select ceil(rn / 5) as gp, ename from t),--我想一页展示五列
5 t2 as
6 (select gp, ename, row_number() over(partition by gp order by ename) as rnn--给每一列编一个序号，便于行转列进行识别
7 from t1)
8 select *
9 FROM t2
10 ;
GP ENAME RNN
---------- ---------- ----------
1 ADAMS 1
1 ALLEN 2
1 BLAKE 3
1 CLARK 4
1 FORD 5
2 JAMES 1
2 JONES 2
2 KING 3
2 MARTIN 4
2 MILLER 5
3 SCOTT 1
3 SMITH 2
3 TURNER 3
3 WARD 4
3 test 5
15 rows selected- 通过分组生成序号,并进行行转列
SQL>
SQL> with t as
2 (select rownum as rn, ename from (select ename from emp order by ename)),--第一步，根据名称进行字段顺序排序，排序后取rownum值，因为我想按照字段顺序打印
3 t1 as
4 (select ceil(rn / 5) as gp, ename from t),--我想一页展示五列
5 t2 as
6 (select gp, ename, row_number() over(partition by gp order by ename) as rnn--给每一列编一个序号，便于行转列进行识别
7 from t1)
8 select *
9 FROM t2
10 pivot (max(ename) as e for rnn in(1 as d1,
11 2 as d2,
12 3 as d3,
13 4 as d4,
14 5 as d5));
GP D1_E D2_E D3_E D4_E D5_E
---------- ---------- ---------- ---------- ---------- ----------
1 ADAMS ALLEN BLAKE CLARK FORD
2 JAMES JONES KING MARTIN MILLER
3 SCOTT SMITH TURNER WARD test有些前台打印功能较弱,就可以用这种办法直接返回需要的数据进行打印。
四、放假安排团队分组值班，如何快速进行人员分组？
现在有个需求：五一五天假期快到了，公司想安排公司成员分组，每天一批进行值班，怎么去快速将人员分成5组呢？
我们可以用分析函数ntile来处理这个分组需求。
SQL> select ntile(5)over(order by empno) as gp,ename from emp;
GP ENAME
---------- ----------
1 test
1 SMITH
1 ALLEN
2 WARD
2 JONES
2 MARTIN
3 BLAKE
3 CLARK
3 SCOTT
4 KING
4 TURNER
4 ADAMS
5 JAMES
5 FORD
5 MILLER
15 rows selected至于怎么分的,这里不进行详细说明了。
总结
本篇文章介绍了4个场景，这四个场景重点介绍了几个函数配合行转列函数的实际使用场景，可以看到，行转列写法有时候可以用来进行SQL优化！~
