常用sql語句

創建數據庫：create database menagerie

建表：create table pet (name varchar(20), owner varchar(20), species varchar(20), sex char(1), birth date, death date)

employee表

pet表

shop表

更新表中名字爲Browser生日： update pet set birth = '1989-08-31' where name = 'Browser'

根據某字段排序：select name, species from pet order by birth desc

查詢日期年、月、日：select Year('2018-02-05') as years from pet
Month()、Day()

過濾某個字段值最大的整條記錄數據：select article, declear, price from pet where price = (select max(price) from shop)

每列最大值，根據某字段分組：select article, max(price) as price from shop group by article

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聚合函數

count、sum、avg、max、min

count(1)、count()、count(列名)區別：
count(1)和count()執行結果同，查詢包括null列數據
count(列名)查詢不包括null列數據

執行效率上：
查詢字段不爲主鍵，count(1)>count(列名)>count(*)
查詢字段爲主鍵，count(列名)最優

內、左、右連接
select * from a inner(left|right) join b on a.A_ID=b.A_ID

in 和 exists
子查詢記錄少，主表較大，有索引：用in優
子查詢表大，有索引，用exists優

select * from a where A_ID in(1,2,3)
select * from a where exists (select A_ID from b)

case sex
when '1' then '男'
when '2' then '女'
else '其他' end

select *, case when salary < 5000 then "低等收入"
when salary >= 5000 and salary < 10000 then "中等收入"
when salary > 10000 then "高等收入" end as level from employee

where 、group by、 having、 order by排序
select name, sum(*) from employee_tb1 where id <> 1 group by name having sum(*) > 5 order by sum(*) desc

刪除、添加、修改表字段、字段類型、表名

alter table a drop
alter table a add i int
alter table a modify c char(10)
alter table a rename to b

常用函數

返回字符串s字符數：select char_length(s)
字符串s1、s2等多個字符串合併一個字符串：select concat(s1,s2) as s
返回第一個字符串s在字符串s1、s2中位置：select field(s1,s2)
返回字符串s的前n個字符：select left(s,n)
首尾去空格：trim(s)
從字符串s截取start位置處長度爲length的字符串：select substring(s, start, length)

索引

創建索引：create index id on b(B_ID)
修改表添加索引：alter table b add index price(B_price)
刪除索引：drop index price on b