1. 结果集分页
1.1. 只有做过了排序，才有可能准确地从结果集中返回指定区间的记录
1.2. DB2
1.3. Oracle
1.4. SQL Server
1.5. sql
复制代码
select sal
from (
select row_number() over (order by sal) as rn,
sal
from emp
) x
where rn between 1 and 5
SAL
----
800
950
1100
1250
1250
1.5.2. sql
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select sal
from (
select row_number() over (order by sal) as rn,
sal
from emp
) x
where rn between 6 and 10
SAL
-----
1300
1500
1600
2450
2850
1.6. PostgreSQL
1.7. MySQL
1.8. sql
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select sal
from emp
order by sal limit 5 offset 0
SAL
------
800
950
1100
1250
1250
1.8.2. sql
复制代码
select sal
from emp
order by sal limit 5 offset 5
SAL
-----
1300
1500
1600
2450
2850
2. 跳过n行记录
2.1. 获得第一个员工、第三个员工，等等
2.2. DB2
2.3. Oracle
2.4. SQL Server
2.5. 使用窗口函数ROW_NUMBER OVER为每一行分配一个序号
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select ename
from (
select row_number() over (order by ename) rn,
ename
from emp
) x
where mod(rn,2) = 1
2.6. PostgreSQL
2.7. MySQL
2.8. 使用标量子查询
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select x.ename
from (
select a.ename,
(select count(*)
from emp b
where b.ename ＜= a.ename) as rn
from emp a
)x
where mod(x.rn,2) = 1
3. 提取最靠前的n行记录
3.1. 基于某种排序方式从结果集中提取出限定数目的记录
3.2. DB2
3.3. Oracle
3.4. SQL Server
3.5. DENSE_RANK函数
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select ename,sal
from (
select ename, sal,
dense_rank() over (order by sal desc) dr
from emp
) x
where dr ＜= 5
3.6. PostgreSQL
3.7. MySQL
3.8. 使用标量子查询
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select ename,sal
from (
select (select count(distinct b.sal)
from emp b
where a.sal ＜= b.sal) as rnk,
a.sal,
a.ename
from emp a
)
where rnk ＜= 5
4. 对结果排序
4.1. DB2
4.2. Oracle
4.3. SQL Server
4.4. 窗口函数DENSE_RANK OVER
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select dense_rank() over(order by sal) rnk, sal
from emp
4.5. PostgreSQL
4.6. MySQL
4.7. 标量子查询
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select (select count(distinct b.sal)
from emp b
where b.sal ＜= a.sal) as rnk,
a.sal
from emp a
5. 删除重复项
5.1. DB2
5.2. Oracle
5.3. SQL Server
5.4. 窗口函数ROW_NUMBER OVER
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select job
from (
select job,
row_number()over(partition by job order by job) rn
from emp
)x
where rn = 1
5.5. PostgreSQL
5.6. MySQL
5.7. sql
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select distinct job
from emp
select job
from emp
group by job
5.7.3. GROUP BY和DISTINCT是两个非常不同的子句，它们是不可互换的
6. 骑士值
6.1. 返回一个结果集，其中包括每个员工的姓名、部门、工资、入职时间以及每一个部门里最近入职的那个员工的工资
6.2. DB2
6.3. SQL Server
6.4. 窗口函数MAX OVER
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select deptno,
ename,
sal,
hiredate,
max(latest_sal)over(partition by deptno) latest_sal
from (
select deptno,
ename,
sal,
hiredate,
case
when hiredate = max(hiredate)over(partition by deptno)
then sal else 0
end latest_sal
from emp
) x
order by 1, 4 desc
6.5. Oracle
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select deptno,
ename,
sal,
hiredate,
max(sal)
keep(dense_rank last order by hiredate)
over(partition by deptno) latest_sal
from emp
order by 1, 4 desc
6.6. PostgreSQL
6.7. MySQL
6.8. 两层嵌套的标量子查询
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select e.deptno,
e.ename,
e.sal,
e.hiredate,
(select max(d.sal)
from emp d
where d.deptno = e.deptno
and d.hiredate =
(select max(f.hiredate)
from emp f
where f.deptno = e.deptno)) as latest_sal
from emp e
order by 1, 4 desc
