Mysql 语句执行顺序

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纸上得来终觉浅

1.这样一个问题，作为一个开发人员需要掌握数据库的哪些东西?  在开发中涉及到数据库，基本上只用到了sql语句，如何写sql以及对其进行优化就比较重要，那些mysql的厚本书籍针对的是DBA，我们只需要学习其中的sql就可以了。

2.既然会写sql是目标，那么怎么才能写好sql.学习下面几点：

1)Mysql的执行顺序，这个是写sql的核心，之前遇到的一些错误就是因为对其不了解；

2)如何进行多表查询，优化，这个是很重要的部分；

3)sql语句的函数，sql提供的函数方便了很多操作；

3.这篇对Mysql语句执行顺序的学习做了总结：

1)Mysql语法顺序，即当sql中存在下面的关键字时，它们要保持这样的顺序：

select[distinct]
from
join（如left join）
on
where
group by
having
union
order by
limit

2)Mysql执行顺序，即在执行时sql按照下面的顺序进行执行：

from
on
join
where
group by
having
select
distinct
union
order by

建立如下表格orders：

注：下面所有语句符合语法顺序(也不可能不符合，因为会报错^_^)，只分析其执行顺序：(join和on属于多表查询，放在最后展示)

语句一：

select a.Customer
from orders a
where a.Customer='Bush' or a.Customer = 'Adams'

语句二groupby：groupby要和聚合函数一起使用

select a.Customer,sum(a.OrderPrice)
from orders a
where a.Customer='Bush' or a.Customer = 'Adams'
group by a.Customer

语句三having：

select a.Customer,sum(a.OrderPrice)
from orders a
where a.Customer='Bush' or a.Customer = 'Adams'
group by a.Customer
having sum(a.OrderPrice) > 2000

语句四distinct： （为测试，先把数据库中Adams那条记录的OrderPrice改为3000）

select distinct sum(a.OrderPrice)
from orders a
where a.Customer='Bush' or a.Customer = 'Adams' or a.Customer = 'Carter'
group by a.Customer
having sum(a.OrderPrice) > 1700

语句五union：完全是对select的结果进行合并（默认去掉重复的记录）：

select distinct sum(a.OrderPrice) As Order1
from orders a
where a.Customer='Bush' or a.Customer = 'Adams' or a.Customer = 'Carter'
group by a.Customer
having sum(a.OrderPrice) > 1500
union
select distinct sum(a.OrderPrice) As Order1
from orders a
where a.Customer='Bush' or a.Customer = 'Adams' or a.Customer = 'Carter'
group by a.Customer
having sum(a.OrderPrice) > 2000

语句六order by：

select distinct sum(a.OrderPrice) As order1
from orders a
where a.Customer='Bush' or a.Customer = 'Adams' or a.Customer = 'Carter'
group by a.Customer
having sum(a.OrderPrice) > 1500
union
select distinct sum(a.OrderPrice) As order1
from orders a
where a.Customer='Bush' or a.Customer = 'Adams' or a.Customer = 'Carter'
group by a.Customer
having sum(a.OrderPrice) > 2000
order by order1

语句七limit：

select distinct sum(a.OrderPrice) As order1
from orders a
where a.Customer='Bush' or a.Customer = 'Adams' or a.Customer = 'Carter'
group by a.Customer
having sum(a.OrderPrice) > 1500
union
select distinct sum(a.OrderPrice) As order1
from orders a
where a.Customer='Bush' or a.Customer = 'Adams' or a.Customer = 'Carter'
group by a.Customer
having sum(a.OrderPrice) > 2000
order by order1
limit 1

语句八（上面基本讲完，下面是join 和 on）：

select distinct sum(a.OrderPrice) As order1,sum(d.OrderPrice) As order2
from orders a
left join (select c.* from Orders c) d 
on a.O_Id = d.O_Id
where a.Customer='Bush' or a.Customer = 'Adams' or a.Customer = 'Carter'
group by a.Customer
having sum(a.OrderPrice) > 1500
union
select distinct sum(a.OrderPrice) As order1,sum(e.OrderPrice) As order2
from orders a
left join (select c.* from Orders c) e 
on a.O_Id = e.O_Id
where a.Customer='Bush' or a.Customer = 'Adams' or a.Customer = 'Carter'
group by a.Customer
having sum(a.OrderPrice) > 2000
order by order1
limit 1

注：

1)使用distinct要写在所有要查询字段的前面，后面有几个字段，就代表修饰几个字段，而不是紧随distinct的字段；

2)group by执行后(有聚合函数)，group by后面的字段在结果中一定是唯一的，也就不需要针对这个字段用distinct；

下篇讲如何进行多表查询

Mysql 语句执行顺序

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原文地址：http://blog.csdn.net/jintao_ma/article/details/51253356