SQL语句的一些基本使用以及一些技巧

#SELECT 列名1， 列名2， from 表明

#SELECT id,title,content,type from news 效率相对较高
#SELECT * from news *代表所有字段，效率相对较低
#SELECT * from news where id = 1 查询出id=1
#SELECT * from news where type = '国际军事类' int类不需要加单引号，varchar需要单引号

#插入语句

#INSERT info 表名（列名1，列名2）
#INSERT into news(title,content, type) values(
#'年度最惊悚的小说','和凤姐同居的日子','国内八卦')

#更新语句
#update 表名 set 列名1=列1值，列名2=列2值 where 判断条件
#UPDATE news set title = '本月最恐怖',content='我和芙蓉同居的日子' where id = 4

#删除语句
#delete from 表名 where 条件
#delete from news where id = 3

外键查询

外键查询

select news.id, news.title,news.content,

newstype.name from news , newstype where
news.type = newstype.id

外键查询简单写法

select n.id,title,content, t.name from news n, newstype t where
n.type = t.id

改变列名

select n.id,title as 标题,content, t.name from news n, newstype t where
n.type = t.id

#内连接写法1
#select * from prisoner, prtype where type = prtype.id

#内连接写法2
#select * from prisoner inner join prtype on
#type = prtype.id

连锁查询

#外连接
select * from prisoner right join prtype on type = prtype.id

#左连接和内连接显示结果一样
select * from prisoner left join prtype on
type = prtype.id

#全连接
select * from prisoner right join prtype on
type = prtype.id
union
select * from prisoner left join prtype on
type = prtype.id

查询出名字以郭开头犯人的名字(like语句)

查询出名字中含有美的犯人 两个百分号 %%

select * from prisoner where name like '%美%'

以编号排序

select * from prisoner order by id

以编号倒序

select * from prisoner order by id desc

找出最小的编号（min语句）

select min(id) from prisoner

找出最大的编号（max语句）

select max(id) from prisoner

查询出所有犯人编号之和，平均数（sum和avg语句）

select sum(id) from prisoner

select avg(id) from prisoner

查询出犯有吸毒罪的人员个数。(count语句)

SELECT count(prisoner.id) from prisoner ,prtype where type = prtype.id and prtype.name = '吸毒类'

查询出所有犯罪类型的犯罪个数(count+group by 语句)

SELECT count(prisoner.id) as 数量,prtype.name from prisoner ,prtype where type = prtype.id
group by prtype.name

查询出前5个犯人的信息(top语句 mysql要用limit)

select * from prisoner limit 0,5

查询出前2个犯人的信息倒叙(top语句 mysql要用limit)

select * from prisoner order by id desc limit 0,2