初学者必读：提高SQL执行效率的几点建议

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初学者必读：提高SQL执行效率的几点建议_MySQL

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Jun 01, 2016 pm 02:00 PM

wherebeginnerInquire

高SQL执行效率的几点建议:

◆尽量不要在where中包含子查询；

关于时间的查询，尽量不要写成：where to_char(dif_date,'yyyy-mm-dd')=to_char('2007-07-01','yyyy-mm-dd')；

◆在过滤条件中，可以过滤掉最大数量记录的条件必须放在where子句的末尾；

FROM子句中写在最后的表（基础表，driving table）将被最先处理，在FROM子句中包含多个表的情况下，你必须选择记录条数最少的表作为基础表。如果有三个以上的连接查询，那就需要选择交叉表（intersection table）作为基础表，交叉表是指那个被其他表所引用的表；

◆采用绑定变量

◆在WHERE中尽量不要使用OR

◆用EXISTS替代IN、用NOT EXISTS替代NOT IN；

◆避免在索引列上使用计算：WHERE SAL*12>25000；

◆用IN来替代OR： WHERE LOC_ID=10 OR LOC_ID=15 OR LOC_ID=20

◆避免在索引列上使用IS NULL和IS NOT NULL；

◆总是使用索引的第一个列；

◆用UNION-ALL替代UNION；

◆避免改变索引列的类型：SELECT...FROM EMP WHERE EMPNO='123'，由于隐式数据类型转换，to_char(EMPNO)='123'，因此，将不采用索引，一般在采用字符串拼凑动态SQL语句出现；

◆'!=' 将不使用索引；

◆优化GROUP BY；

◆避免带有LIKE参数的通配符，LIKE '4YE%'使用索引，但LIKE '%YE'不使用索引

◆避免使用困难的正规表达式，例如select * from customer where zipcode like "98___"，即便在zipcode上建立了索引，在这种情况下也还是采用顺序扫描的方式。如果把语句改成select * from customer where zipcode>"98000"，在执行查询时就会利用索引来查询，显然会大大提高速度；

◆尽量明确的完成SQL语句，尽量少让数据库工作。比如写SELECT语句时，需要把查询的字段明确指出表名。尽量不要使用SELECT *语句。组织SQL语句的时候，尽量按照数据库的习惯进行组织。

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