Mysql的DQL查詢操作全面分析講解

更新時間：2022年12月03日 10:01:35 作者：coleak

DQL（Data Query Language 數(shù)據(jù)查詢語言）：用于查詢數(shù)據(jù)庫對象中所包含的數(shù)據(jù)。DQL語言主要的語句：SELECT語句。DQL語言是數(shù)據(jù)庫語言中最核心、最重要的語句，也是使用頻率最高的語句

DQL簡介

概念：DQL（data query language）數(shù)據(jù)查詢語言 select操作

排序規(guī)則：

- select 表達式1|字段,.... - from 表名 where 條件 - group by 列名 - having 條件 - order by 列名 asc|desc - limit 位置,數(shù)量

語法結構：

SELECT [ALL | DISTINCT] ALL表示查詢出所有的內容 DISTINCT 去重
{* | 表名.* | 表名.字段名[ AS 別名][,...]} 指定查詢出的字段的
FROM
表名[AS 別名][,表1... AS 別名]
[INNER | [LEFT | RIGHT] [OUTER] JOIN 另一張表名 [AS 別名] ON 關聯(lián)條件]
[WHERE 條件]
[GROUP BY 分組字段[,...]]
[HAVING 給分組后的數(shù)據(jù)進行條件篩選]
[ORDER BY 排序字段[,...]]
[LIMIT [startIndex,]pageSize]

具體操作

數(shù)據(jù)準備

create database if not exists test;
use test;
create table if not exists data(
id tinyint primary key auto_increment,
price double NOT null,
name varchar(20) not null,
type varchar(20) not null)
;
insert into data values
(null,900,'洗衣機','b'),
(null,1900,'冰箱','b'),
(null,2900,'空調','b'),
(null,3900,'電視','b'),
(null,150,'衣服','c'),
(null,180,'褲子','c'),
(null,200,'鞋子','c'),
(null,188,'洗面奶','a'),
(null,188,'洗發(fā)水','a'),
(null,199,'洗衣液','a'),
(null,88,'沐浴露','a'),
(null,5,'泡面','d'),
(null,15,'餅干','d'),
(null,30,'咖啡','d');

簡單查詢

select * from data;
select name,price from data;
select * from data as d;
select * from data d;
select d.name,d.price from data d;
select  distinct price from data;
select name,price +100 newprice from data;

運算符

算術運算符

select name,price *1.5 newprice from data;

條件查詢

select * from data where name='洗衣機';
select * from data where  !(price>100);
select * from data where price between 200 and 1000;
select * from data where price in(188,900);
-- 等于下面兩句
select * from data where price = 188 or price =900;
select * from data where price = 188 || price =900;
select * from data where name like '%衣%';
select * from data where name like '衣%';
select * from data where name like '_衣%';
select * from data where id is null;

注釋：當有NULL作為比較大小的對象時，最大值和最小值均為null

排序查詢

select * from data order by price;
select * from data order by price desc;
select distinct price from data order by price desc;
select * from data order by price,id;

聚合查詢

select count(*) from data;
-- 不全為空的行數(shù)
select count(id) from data;
-- 通過主鍵值查詢行數(shù)
select count(*) from data where price<200;
select sum(price) from data where type='A';
select max(id) from data;
select min(price) from data;
select max(price) max_price,min(price) min_price from data;
select avg(price) from data where type='c';

null值的處理

分組查詢

select sum(price) from data group by type;
select type,count(id) from data group by type;

條件篩選

select type,count(id) count from data group by type having count=4 order by type;

分頁查詢

分頁顯示

select * from data limit 5;
-- 從第四條開始依次向后顯示五條
select * from data limit 3,5;

insert into select語句

create table data2(
name varchar(10),
price double);
insert into data2 select name,price from data;
select * from data2;
create table data3(
type varchar(10),
num int
);
insert into data3 select type,count(*) from data group by type order by count(*);
select * from data3;