--數(shù)據(jù)庫操作前的準(zhǔn)備
-- 創(chuàng)建數(shù)據(jù)庫
-- create database python_test_1 charset=utf8;
-- 使用數(shù)據(jù)庫
-- use python_test_1;
-- students表
-- create table students(
-- id int unsigned primary key auto_increment not null,
-- name varchar(20) default '',
-- age tinyint unsigned default 0,
-- height decimal(5,2),
-- gender enum('男','女','中性','保密') default '保密',
-- cls_id int unsigned default 0,
-- is_delete bit default 0
-- );
-- classes表
-- create table classes (
-- id int unsigned auto_increment primary key not null,
-- name varchar(30) not null
-- );
-- 查詢練習(xí)
-- 查詢所有字段
-- select * from 表名;
select * from students;
-- 查詢指定字段
-- select 列1,列2,... from 表名;
select name,age from students;
-- 使用 as 給字段起別名
-- select 字段 as 名字.... from 表名;
select name as '姓名',age from students;
-- select 表名.字段 .... from 表名;
select students.name from students;
-- 可以通過 as 給表起別名
-- select 別名.字段 .... from 表名 as 別名;
select * from students as s;
select s.name from students as s;
-- 消除重復(fù)行(查性別)
-- distinct 字段 不要記有個印象
select distinct gender from students;
> < >= <= != <>
-- 條件查詢
-- 比較運算符
-- select .... from 表名 where .....
-- >
-- 查詢年紀(jì)大于18歲的信息
select * from students where age > 18;
-- <
-- 查詢年紀(jì)小于18歲的信息
select * from students where age < 18;
-- >=
-- <=
-- 查詢小于等于18歲的信息
select * from students where age <= 18;
-- =
-- 查詢年齡為18歲的所有學(xué)生的名字
select * from students where age = 18;
-- != 或者 <>
-- 查詢年齡不為18歲的所有學(xué)生的名字
select * from students where age != 18;
-- select * from students where age <> 18;
and or not
-- 邏輯運算符
-- and
-- 18和28之間的所以學(xué)生信息
select * from students where age > 18 and age < 28;
-- 18歲以上的女性
select * from students where age > 18 and gender = '女';
-- or
-- 18以上或者身高高過180(包含)以上
select * from students where age > 18 or height >= 180;
-- not
-- 不在 18歲以上的女性 這個范圍內(nèi)的信息
-- select * from students where not (age>18 and gender=2);
select * from students where not age > 18 and gender= "女";
select * from students where not (age > 18 and gender = "女");
like % _
-- 模糊查詢(where name like 要查詢的數(shù)據(jù))
-- like
-- % 替換任意個
-- _ 替換1個
-- 查詢姓名中 以 "小" 開始的名字
select * from students where name like '小%';
-- 查詢姓名中 有 "小" 所有的名字
select * from students where name like '%小%';
-- 查詢有2個字的名字
select * from students where name like '__';
-- 查詢有3個字的名字
select * from students where name like '___';
-- 查詢至少有2個字的名字
select * from students where name like '__%';
select * from students where name not like "__";
-- 范圍查詢
-- in (1, 3, 8)表示在一個非連續(xù)的范圍內(nèi)
-- 查詢 年齡為18或34的姓名
select * from students where age = 18 or age = 34 ;
select * from students where age in (18,34);
-- not in 不非連續(xù)的范圍之內(nèi)
-- 年齡不是 18或34歲的信息
select * from students where age not in(18,34);
-- between ... and ...表示在一個連續(xù)的范圍內(nèi)
-- 查詢 年齡在18到34之間的的信息
select * from students where age > 18 and age < 34;
-- between xxx and xxx
select * from students where age between 18 and 34; --between...and...這是包含兩端的數(shù)據(jù)
-- not between ... and ...表示不在一個連續(xù)的范圍內(nèi)
-- 查詢 年齡不在18到34之間的的信息
select * from students where age not between 18 and 34;
-- 空判斷
-- 判空is null
-- 查詢身高為空的信息
select * from students where height is null;
-- 判非空is not null
select * from students where height is not null;
order by 字段 asc,desc
-- 排序
-- order by 字段
-- asc
-- asc從小到大排列，即升序
-- desc
-- desc從大到小排序，即降序
-- 查詢年齡在18到34歲之間的男性害淤，按照年齡從小到大到排序
select * from students where (age between 18 and 34) and gender='男' order by age asc;
-- 查詢年齡在18到34歲之間的女性巨朦，身高從高到矮排序
select * from students where (age between 18 and 34) and gender ='女' order by height desc;
-- order by 多個字段
-- 查詢年齡在18到34歲之間的女性预茄，身高從高到矮排序, 如果身高相同的情況下按照年齡從小到大排序
select * from students where (age between 18 and 34) and gender = '女' order by height desc,age asc;
-- 如果年齡也相同那么按照id從大到小排序
select * from students where (age between 18 and 34) and gender ='女' order by height desc,age asc,id desc;
-- 排序有優(yōu)先級,第一個主排序,后面是次排序,在保證主排序不變的情況下,能排就排,不排就算了
-- 聚合函數(shù)
-- 總數(shù)
-- count
-- 查詢男性有多少人 count(字段) 要注意如果值有null那么不會進(jìn)行計算
select count(*) from students where gender='男';
-- 最大值
-- max
-- 查詢最大的年齡
select max(age) from students;
-- 查詢女性的最高 身高
select max(height) from students where gender ='女';
-- 最小值
-- min
select min(age) from students ;
-- 求和
-- sum
-- 計算所有人的年齡總和
select sum(age) from students;
-- 平均值
-- avg
-- 計算平均年齡
select avg(age) from students;
-- 計算平均年齡 sum(age)/count()
select sum(age)/count() from students;
-- 四舍五入 round(123.23 , 1) 保留1位小數(shù)
-- 計算所有人的平均年齡狠怨，保留2位小數(shù)
select round (avg(age),2) from students;
-- 計算男性的平均身高 保留2位小數(shù)
select round(avg(height),2) from students where gender='男';
select avg(height) from students where gender = '男';
-- 分組
-- group by
-- 按照性別分組,查詢所有的性別
-- select 分組字段 from 表名 group by 分組字段;
select gender from students group by gender;
select 分組字段 from 表名 group by 分組字段;
-- 計算每種性別中的人數(shù)
select gender,count(*) from students group by gender;
-- group_concat(...)
-- 查詢同種性別中的姓名
select gender,group_concat(name) from students group by gender;
-- 查詢每組性別的平均年齡
select gender,avg(age) from students group by gender;
-- select * from students where
-- group by xxx having having用在分組條件
-- having(注意having和group by 連用 having后通常也要跟 聚合函數(shù))
-- 查詢平均年齡超過30歲的性別坑资，以及姓名
select gender ,avg(age) from students group by gender having avg(age) > 30;
-- 查詢每種性別中的人數(shù)多于2個的信息
select gender,count() from students group by gender having count() > 2;
-- with rollup 匯總的作用(了解)
--select gender,count() from students group by gender with rollup;
select gender,count() from students group by gender with rollup having count(*) >2;
--按性別分組,平均身高大160的女性組的名字
select gender,avg(height),group_concat(name) from students group by gender having avg(height) > 160 and gender='女';
-- limit 起始位置,個數(shù), 這個一定要放在最后
-- 分頁
-- limit start, count
-- limit 放在最后面(注意)
起始位置 = (頁數(shù)-1)*每頁的個數(shù)
-- 限制查詢出來的數(shù)據(jù)個數(shù)
-- 查詢前5個數(shù)據(jù)
select * from students limit 0,5;
-- 每頁顯示2個运敢，第1個頁面
select * from students limit 0,2;
-- 每頁顯示2個校仑，第2個頁面
select * from students limit 2,2;
-- 每頁顯示2個，第3個頁面
select * from students limit 4,2;
-- 每頁顯示2個传惠，第4個頁面
select * from students limit 6,2;
-- 每頁顯示2個迄沫，顯示第6頁的信息, 按照年齡從小到大排序
select * from students order by age asc limit 6,2;
-- 如果重新排序了,那么會顯示第一頁
-- 連接查詢
-- inner join ... on
-- select ... from 表A inner join 表B;
-- 查詢 有能夠?qū)?yīng)班級的學(xué)生以及班級信息
select * from students inner join classes on students.cls_id = classes.id;
-- 按照要求顯示姓名、班級
select students.name,classes.name from students inner join classes on students.cls_id = classes.id;
-- 給數(shù)據(jù)表起名字
select s.name,c.name from students as s inner join classes as c on s.cls_id = c.id;
-- 查詢 有能夠?qū)?yīng)班級的學(xué)生以及班級信息卦方，顯示學(xué)生的所有信息 students.羊瘩，只顯示班級名稱 classes.name.
select students. ,classes.name from students inner join classes on students.cls_id = classes.id;
-- 在以上的查詢中，將班級名顯示在第1列
select classes.name,students.* from students inner join classes on students.cls_id = classes.id;
-- 查詢 有能夠?qū)?yīng)班級的學(xué)生以及班級信息, 按照班級名進(jìn)行排序
select classes.name,students.* from students inner join classes on students.cls_id = classes.id order by classes.name asc;
-- 當(dāng)時同一個班級的時候盼砍，按照學(xué)生的id進(jìn)行從小到大排序
select classes.name,students.* from students inner join classes on students.cls_id = classes.id order by classes.name asc,students.id asc;
#如果是group by 條件使用having
#如果是inner join條件使用on
#其他都用where
-- left join
-- 查詢每位學(xué)生對應(yīng)的班級信息
select * from students left join classes on students.cls_id = classes.id;
左邊的表不管在右邊的表中是否找到數(shù)據(jù),都顯示
-- 查詢沒有對應(yīng)班級信息的學(xué)生
select * from students left join classes on students.cls_id = classes.id where classes.name is null;
-- right join on
-- 將數(shù)據(jù)表名字互換位置尘吗，用left join完成
select * from students right join classes on students.cls_id = classes.id;
select * from classes right join students on students.cls_id = classes.id;
-- 子查詢
-- 標(biāo)量子查詢: 子查詢返回的結(jié)果是一個數(shù)據(jù)(一行一列)
-- 列子查詢: 返回的結(jié)果是一列(一列多行)
-- 行子查詢: 返回的結(jié)果是一行(一行多列)
-- 查詢出高于平均身高的信息(height)
select avg(height) from students;
select * from students where height > 172;
select * from students where height > (select avg(height) from students);
-- 查詢學(xué)生的班級號能夠?qū)?yīng)的 學(xué)生名字
select * from students where cls_id in (1,2);
select id from classes;
select * from students where cls_id in (select id from classes);
省市區(qū)三級聯(lián)動
--數(shù)據(jù)操作前的準(zhǔn)備
--創(chuàng)建數(shù)據(jù)庫表
create table areas(
aid int primary key,
atitle varchar(20),
pid int
);
--從sql文件中導(dǎo)入數(shù)據(jù)
-- source 具體地址/areas.sql;
source areas.sql;
--查詢一共有多少個省
select * from areas where pid is null;
--例1：查詢省的名稱為“山西省”的所有城市
select aid from areas where atitle = '山西省';
select * from areas where pid = (select aid from areas where atitle = '山西省');
select * from areas as a1 inner join areas as a2 on a1.pid = a2.aid where a2.atitle='山西省';
--例2：查詢市的名稱為“廣州市”的所有區(qū)縣
select * from areas where pid = (select aid from areas where atitle = '廣州市');
select * from areas as a1 inner join areas as a2 on a1.pid = a2.aid where a2.atitle='廣州市';
