不同数据库 selecto top N 的用法

1. ORACLE 
SELECT * FROM TABLE1 WHERE ROWNUM<=N

2. INFORMIX 
SELECT FIRST N * FROM TABLE1

3. DB2 
SELECT * ROW_NUMBER() OVER(ORDER BY COL1 DESC) AS ROWNUM WHERE ROWNUM<=N 
或者
SELECT COLUMN FROM TABLE FETCH FIRST N ROWS ONLY 

4. SQL SERVER 
SELECT TOP N * FROM TABLE1 

5. SYBASE
SET ROWCOUNT N GOSELECT * FROM TABLE1 

6. MYSQL 
SELECT * FROM TABLE1 LIMIT N 

7. FOXPRO 
SELECT * TOP N FROM TABLE ORDER BY COLUMN

having 和 where 的区别：

1. where 子句中不能有聚组函数(sum,count,avg,max)
2. 一般group by 之前用where ， 之后用having。

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插入特殊符号，如单引号，单引号会 断掉sql 语句

sql = insert into yourTable(f1,f2) values(100,'abc') 

这时一般用两个单引号

replace(sql,"'","''")

& 是 chr(38)

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union and union all usage:

SELECT column_name(s) FROM table_name1
UNION
SELECT column_name(s) FROM table_name2

it will show the two tables ** column_name values ** (column_name should be the same) without
duplicate.

SELECT column_name(s) FROM table_name1
UNION ALL
SELECT column_name(s) FROM table_name2

union all will show the duplicate data

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http://coolshell.cn/articles/1846.html

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2017.2.21 --- left join 优化思考

processing order of the select statement
(Select statement 执行的顺序)：
1.FROM

2.ON

3.JOIN

4.WHERE

5.GROUP BY

6.WITH CUBE or WITH ROLLUP

7.HAVING

8.SELECT

9.DISTINCT

10.ORDER BY

11.TOP

on 在 where 之后执行，说明join 的时候在 on 过滤比在where过滤效率高

select * from a left join b on a.name = b.name and a.group = 1
select * from a left join b on a.name = b.name where a.group = 1

http://luxuryzh.iteye.com/blog/1976004

http://lucien-zzy.iteye.com/blog/2308137