我們先把數(shù)據(jù)表建好
use test;
create table `employee`(
emp_no int unsigned,
emp_name varchar(30),
emp_sex varchar(3),
emp_age tinyint unsigned,
sal double,
history datetime
);
insert into employee values(1, '張三', '男', 18, 5000, '2012-04-23'),
(2, '李四', '男', 27, 4500, '2013-05-23'),
(3, '王五', '男', 23, 4700, '2012-04-21'),
(4, '子龍', '男', 19, 3800, '2011-03-04'),
(5, '李白', '男', 15, 6200, '2015-09-09'),
(6, '劉備', '男', 28, 2500, '2016-02-11'),
(7, '呂布', '男', 21, 6000, '2010-10-18'),
(8, '尚香', '女', 16, 4500, '2011-09-26'),
(9, '小喬', '女', 15, null, '2013-07-05'),
(10, '大喬', '女', 16, 5000, '2017-09-01');
常用的運算符:
1: 等于( = )
select * from employee where sal = 3800;
select * from employee where sal = null; --這里查詢不到為null的數(shù)據(jù)
2: 等于( => )
select * from employee where sal => 3800;
select * from employee where sal => null; --這里可以查詢到為null的數(shù)據(jù)
3: is判斷(null)
select * from employee where sal is null;
select * from employee where sal is not null;
4: null值判斷還可以使用isnull();
select * from employee where isnull(sal);
select * from employee where !isnull(sal);
5: 在區(qū)間(between)內(nèi) between min and max ps:這里是一個閉區(qū)間
select * from employee where sal between 4500 and 5000;
6: 不在區(qū)間內(nèi)
select * from employee where sal not between 4500 and 5000; --null不為包括進去
7: and 和 or
select * from employee where sal not between 4500 and 5000 or sal is null;
select * from employee where sal = 4500 and emp_sex = '女';
8: 小于(), 大于(>), 小于等于(=), 大于等于(>=)
select * from employee where sal >= 4500;
***************************************************************************************************************
數(shù)學(xué)函數(shù)
1: rand();
select rand() from dual; --dual是一個偽表
select 1+1 from dual;
select rand(); --可以簡寫
2: least(value1, value2, ...) 返回最小值
select least(54,76,4,65,76,87,87,56,65,654,45,23,1,76);
select least(54,76,4,65,76,87,87,56,65,654,45,23,1,76) as min_value; --列名可以起一個別名
3: greatest(value1, value2, ...) 返回最大值
select greatest(54,76,4,65,76,87,87,56,65,654,45,23,1,76);
4: round(M, D); 返回M的四舍五入的值, D表示要保留幾們小數(shù),默認值是0
select round(1.69);
select round(1.69, 1);
5: abs() 絕對值
select 5-10;
select abs(5-10);
***************************************************************************************************************
匯總函數(shù)
1: avg();
select * from employee where sal >= 6000;
select avg(sal) from employee where sal >= 6000;
2: count()
select count(*) from employee;
select count(emp_name) from employee;
select count(sal) from employee; --打印9 這里會忽略null值
select count(*) from employee where sal >= 4000;
select count(*) from employee where sal = 4000 or sal is null;
3: sum()
select sum(sal) from employee where sal >= 6000;
4: min()
select min(sal) from employee;
5: max()
select max(sal) from employee;
***************************************************************************************************************
日期函數(shù)
1: 獲取當(dāng)前的日期時間
select now(), sysdate(), current_timestamp();
select now(6), sysdate(6), current_timestamp(6);
ps: now(), current_timestamp();沒有區(qū)別, 表示sql開始執(zhí)行時的時間
sysdate()表示這個函數(shù)開始時間
2: 獲取當(dāng)前日期
select curdate(); --只有年月日
3: 獲取當(dāng)前時間
select curtime(); --只有時分秒
4: 日期的加運算date_add
select history, date_add(history, interval '1 12:10' day_minute) from employee; --date_add(history, interval '1 12:10' day_minute)
select history, date_add(history, interval '1-1' year_month) from employee; --date_add(history, interval '1-1' year_month)
select history, date_add(history, interval '1' second) from employee; --date_add(history, interval '1' second)
5: 日期的減運算data_sub
select history, date_sub(history, interval '1-1' year_month) from employee;
6: 計算日期差
select history, sysdate(), datediff(sysdate(), history) from employee; --以天數(shù)來表示
7: 獲取日期的指定部分(把日期轉(zhuǎn)換為指定的格式) date_format()
select history, date_format(history, '%Y年%m月%d號') from employee;
select history, date_format(history, '%d號') from employee;
select history, date_format(history, '%Y年%m月%d號 %H時%i分%s秒') from employee;
8: 計算出一個日期是星期幾
select history, dayname(history) from employee;
9: 中文日期字符串轉(zhuǎn)換日期str_to_date()
insert into employee values(11, '張飛', '男', 22, 3000, '2017年02月01號'); --報錯
insert into employee values(11, '張飛', '男', 22, 3000, str_to_date('2017年02月01號', '%Y年%m月%d號 %H時%i分%s秒'));
insert into employee values(12, '二哥', '男', 22, 3000, str_to_date('2017年02月01號 23時02分02秒', '%Y年%m月%d號 %H時%i分%s秒'));
insert into employee values(12, '二哥', '男', 22, 3000, str_to_date('2017年02月01號 11時02分02秒', '%Y年%m月%d號 %h時%i分%s秒'));
ps: 如果是h則表示12小制, 如果是大H則表示24小明制;
字符串函數(shù)
1: left(str, len) 返回字符串str的左端len個字符
select left('abcdefg', 5);
2: length()
select length('abcdefg');
3: lower(str) 返回小寫的字符串str
select lower('HELLO');
4: substring() 取子字符串, 第二個參數(shù)是截取的起始位置, 第三個參數(shù)是要截取的長度
select substring('helloworld',2,3);
5: concat() 字符串拼接
select concat(emp_name, '員工') from employee;
6: replace(替換
select replace(emp_name, '李', '老') from employee where emp_name = '李四';
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