1. 查询技巧

查询时先写出查询结构,将需要连接的表进行join on 连接好，最后再写出查询的数据

select

from

join

on

[where]

group by 字段[order by 字段 asc/desc]

[having]

实例：

# 显示列表包含电影名、用户性别、评分

select a.title, c.gender, b.ratings

from movies a

inner join ratings b

on a.movie_id = b.movie_id

inner join users c

on b.user_id = c.user_id

limit 5;

# 显示列表包含电影名、用户性别、评分 (只包含男性)

select a.title, c.gender, b.ratings

from movies a

inner join ratings b

on a.movie_id = b.movie_id

inner join users c

on b.user_id = c.user_id

where c.gender = 'M' limit 10;

# 显示男女平均打分，只有两行记录

select a.gender, avg(b.ratings)

from users a

inner join ratings b

on a.user_id = b.user_id

group by a.gender;

# 显示每部电影平均打分，电影名，评分

select a.title, avg(b.ratings)

from movies a

inner join ratings b

on a.movie_id = b.movie_id

group by a.title;

# 显示每部电影不同性别平均分

select c.title, a.gender, avg(b.ratings)

from users a

inner join ratings b

on a.user_id = b.user_id

inner join movies c

on b.movie_id = c.movie_id

group by c.title, a.gender;

# 显示电影平均分大于4分的电影

select a.title, avg(b.ratings)

from movies a

inner join ratings b

on a.movie_id = b.movie_id

group by a.title

having avg(b.ratings) > 4;

# 显示电影平均分大于4分的电影(子查询)

#显示电影平均分大于4分的电影(子查询)

select title

from movies

where movieid

in(

select movieid

from movies

where movieid

in

(

select movieid

from ratings

group by movieid

having avg(rating)>4

)

);

# 不同年龄分组电影平均打分

select c.age_min, c.age_max, avg(a.ratings)

from ratings a

inner join users b

on a.user_id = b.user_id

inner join age_dict c

on b.age = c.age_id

group by c.age_min, c.age_max;

数据查询优化: 1. 给当前查询数据添加索引create index ix_user_id on users (user_id)

2.给关系数据库添加主外键关联。

2. mysql数据库表的横纵交换。

create table b

(

姓名 varchar(20),

语文 int,

数学 int,

英语 int

);

insert into b(姓名,语文,数学,英语) values('张三',60,70,80);

insert into b(姓名,语文,数学,英语) values('李四',90,100,0);

insert into b(姓名,语文,数学,英语) values('五狗',80,90,70);

# 加了引号代表每一行都要显示该数据 as 只执行一次(适用于表头)，不加引号代表查询的结果字段

select 姓名, '语文' as 课程, 语文 as 成绩 from b

union all

select 姓名, '数学' as 课程, 数学 as 成绩 from b

union all

select 姓名, '英语' as 课程, 英语 as 成绩 from b

;

# 创建纵表

create table c

select 姓名, '语文' as 课程, 语文 as 成绩 from b

union all

select 姓名, '数学' as 课程, 数学 as 成绩 from b

union all

select 姓名, '英语' as 课程, 英语 as 成绩 from b

;

select * from c;

#1)case vlaue when [compare-value]then reslut [when[compare-value]] then result ...] [else result] end

#(2)case when [condition] then result [when[condition]then result...][else result] end

#第一形式当value=compare-value时返回result

#

#第二形式当第一个为真值的condition出现时，返回该条件的结果，如果没有匹配的结果值，那么else后的结

#果将被返回，如果没有else部分，那么返回null

# 只要出现聚合函数就一定有group by。

select 姓名,

max(case 课程 when '语文' then 成绩 else 0 end) as '语文',

max(case 课程 when '数学' then 成绩 else 0 end) as '数学',

max(case 课程 when '英语' then 成绩 else 0 end) as '英语'

from c

group by 姓名;

select * from c where 课程='数学' ORDER BY 成绩;