1. 在WHERE子句中引用别名列

1.1. 当表里的某些列没有被恰当命名的时候，这个技巧尤其有用

1.2. sql

select sal as salary, comm as commission
  from emp
 where salary ＜ 5000

1.3. 内嵌视图

1.3.1.  sql

select *
   from (
 select sal as salary, comm as commission
   from emp
        ) x
  where salary ＜ 5000

1.3.2. 聚合函数

1.3.3. 标量子查询

1.3.4. 窗口函数

1.3.5. 别名

2. CONCAT函数

2.1. 串联多列的值

2.2. 在DB2、Oracle和PostgreSQL 中，“||”是CONCAT函数的快捷方式

2.3. 在SQL Server中则为“+”

3. 随机返回若干行记录

3.1. 如果ORDER BY子句使用数值常量，那么就需要按照SELECT列表里的顺序来排序

3.2. 如果ORDER BY子句使用了函数，那么就需要按照该函数的返回值来排序，而函数返回的值是根据结果集里的每一行计算而来的

3.3. MySQL

3.3.1.  sql

  select ename,job
 from emp
  order by rand() limit 5

3.4. PostgreSQL

3.4.1.  sql

 select ename,job
   from emp
  order by random() limit 5

3.5. Oracle

3.5.1.  sql

 select *
   from (
  select ename, job
    from emp
   order by dbms_random.value()
        )
   where rownum ＜= 5

3.6. SQL Server

3.6.1.  sql

select top 5 ename,job
   from emp
  order by newid()

4. 把Null值转换为实际值

4.1. COALESCE函数

4.1.1.  sql

select coalesce(comm,0)
   from emp

4.1.2. 更方便、更简洁

4.2. CASE

4.2.1. sql

 select case
       when comm is not null then comm
       else 0
       end
  from emp

4.3. 适用于所有的数据库

5. 依据子串排序

5.1. 按照职位字段的最后两个字符对检索结果进行排序

5.2. DB2、MySQL、Oracle和PostgreSQL

5.2.1. sql

select ename,job
  from emp
 order by substr(job,length(job)-2)

5.3. SQL Server

5.3.1. sql

select ename,job
  from emp
 order by substring(job,len(job)-2,2)

6. 排序时对Null值的处理

6.1. Oracle能够让你在无须修改非Null值数据的情况下方便地把Null值排到最前面或者最后面，其他数据库得添加一个辅助列

6.1.1. Oracle 9i及后续版本NULLS FIRST和NULLS LAST来决定Null值应该排到前面还是后面

6.2. 辅助列

6.2.1. 只存在于查询语句里，而不存在于表中

6.2.2. 目的是让你能够识别出Null值，并控制其排在最前面还是最后面

6.3. 对于非Oracle解决方案的查询语句，其内嵌视图X会返回如下结果集

6.4. sql

select ename,sal,comm,
       case when comm is null then 0 else 1 end as is_null
  from emp

6.5. ENAME    SAL       COMM    IS_NULL

SMITH    800                           0
ALLEN   1600          300          1
WARD    1250         500          1
JONES   2975                          0
MARTIN  1250       1400         1
BLAKE   2850                          0
CLARK   2450                          0
SCOTT   3000                          0
KING    5000                           0
TURNER  1500          0           1
ADAMS   1100                       0
JAMES    950                          0
FORD    3000                         0
MILLER  1300                        0

7.  依据条件逻辑动态调整排序项

7.1. 如果JOB等于SALESMAN，就要按照COMM来排序；否则，按照SAL排序

7.2.  sql

select ename,sal,job,comm
   from emp
  order by case when job = 'SALESMAN' then comm else sal end

7.3. sql

select ename,sal,job,comm,
       case when job = 'SALESMAN' then comm else sal end as ordered
 from emp
order by 5

7.4. ENAME             SAL JOB             COMM    ORDERED

TURNER           1500 SALESMAN           0          0
ALLEN            1600 SALESMAN         300        300
WARD             1250 SALESMAN         500        500
SMITH             800 CLERK                                800
JAMES             950 CLERK                                950
ADAMS            1100 CLERK                             1100
MARTIN           1250 SALESMAN        1300    1300
MILLER           1300 CLERK                               1300
CLARK            2450 MANAGER                        2450
BLAKE            2850 MANAGER                        2850
JONES            2975 MANAGER                        2975
SCOTT            3000 ANALYST                          3000
FORD             3000 ANALYST                           3000
KING             5000 PRESIDENT                        5000