SQL慢

性能下降SQL慢

执行时间长

等待时间长

1. 查询语句太烂
2. 索引失效
3. 单值 只用了一个字段建立索引
4. 复合 用了两个以上的字段建立了索引
5. 关联查询太多
6. 服务器调优及各个参数设置

常见通用的join查询

SQL执行顺序

手写

机读

总结

Join图

建表SQL

7种Join

select [select_list] from table_a a inner join table_b b on a.key = b.key;

select [select_list] from table_a a left join table_b b on a.key = b.key;

select [select_list] from table_a a right join table_b b on a.key = b.key;

select [select_list] from table_a a left join table_b b on a.key = b.key where b.key is null;

select [select_list] from table_a a right join table_b b on a.key = b.key where a.key is null;

select [select_list] from table_a a full outer join table_b b on a.key = b.key;

select [select_list] from table_a a full outer join table_b b on a.key = b.key where a.key is null or b.key is null;

select [select_list] from table_a left join table_b b on a.key = b.key 
union
select [select_list] from table_a right join table_b b on a.key = b.key;

select [select_list] from table_a a left join table_b b on a.key = b.key where b.key is null 
union 
select [select_list] from table_a a right join table_b b on a.key = b.key where a.key is null;