sql常用+sql优化方面总结

sql没学好，补充补充

1.查询前几条记录SQL在不同数据库中的用法

1. ORACLESELECT * FROM TABLE1 WHERE ROWNUM<=N2. INFORMIXSELECT FIRST N * FROM TABLE13. DB2SELECT * ROW_NUMBER() OVER(ORDER BY COL1 DESC) AS ROWNUM WHERE ROWNUM<=N或者SELECT COLUMN FROM TABLE FETCH FIRST N ROWS ONLY4. SQL SERVERSELECT TOP N * FROM TABLE15. SYBASESET ROWCOUNT NGOSELECT * FROM TABLE16. MYSQLSELECT * FROM TABLE1 LIMIT N7. FOXPROSELECT * TOP N FROM TABLE ORDER BY COLUM

2常用SQL基础

http://www.cnblogs.com/hoojo/archive/2011/07/16/2108129.html；

有空就多看看

//gmtdate:15.07.09

exists与in的区别：

IN适合于外表大而内表小的情况；EXISTS适合于外表小而内表大的情况

详细比较，查看链接：http://www.cnblogs.com/eva_2010/articles/2047558.html