SELECT UPPER('Hello');

SELECT LOWER('Hello');

SELECT LENGTH('叶绿体不忘呼吸aaaa');

SELECT CONCAT('Hello','世界') AS result;

#从第4个开始截取，截取后面全部
SELECT SUBSTR('我相信光',4);

#从第1个开始截取，截取3个
SELECT SUBSTR('我相信光',1,3);

SELECT INSTR('国足10月13日客战沙特','沙特') AS result;

SELECT TRIM('a' FROM 'aaaaa叶aa绿体aaaaa') AS result;

#去空格
SELECT TRIM('    叶aa绿体a    ') AS result;

SELECT LPAD('叶绿体',9,'a') AS result;

SELECT REPLACE('a叶aaa绿体aaaa','a','B') AS result;

#默认保留一位小数
SELECT ROUND(1.62) AS result;

#保留两位小数
SELECT ROUND(1.627,2) AS result;

SELECT CEIL(1.002) AS result;

SELECT FLOOR(1.002) AS result;

SELECT TRUNCATE(1.699,1) AS result;

SELECT MOD(10,-3) AS result;

SELECT NOW();

SELECT CURDATE();

SELECT CURTIME();

SELECT YEAR(NOW());

SELECT YEAR('2021-09-30') AS result;

SELECT MONTH('2021-09-30') AS result;

SELECT MONTHNAME('2021-09-30') AS result;

#相当于是解析：两个参数格式要匹配
SELECT STR_TO_DATE('9-30 2021','%m-%d %Y') AS result;

#相当于是格式化
SELECT DATE_FORMAT(NOW(),'%Y年%m月%d日') AS result;

#查看版本
SELECT VERSION();
#查看数据库
SELECT DATABASE();
#查看用户
SELECT USER();

SELECT IF(10>5,'大','小') AS result;

CASE 要判断的
WHEN 常量1 THEN 语句1;或者要显示的值1
...
ELSE 语句;或者要显示的值
END

#示例为要显示的值，不加‘;'
SELECT `last_name`,`salary`,`department_id`,
CASE `department_id`
WHEN 100 THEN `salary`*(1+0.8)
WHEN 90 THEN `salary`*(1+0.6)
WHEN 80 THEN `salary`*(1+0.4)
WHEN 70 THEN `salary`*(1+0.2)
ELSE `salary`
END AS 最终工资
FROM employees;

CASE 
WHEN 条件1 THEN SELECT 语句1;或者要显示的值1
...
ELSE 语句;或者要显示的值
END

#示例为要显示的值，不加‘;'
SELECT `last_name`,`salary`,
CASE
WHEN `salary`>20000 THEN 'A级别'
WHEN `salary`>15000 THEN 'B级别'
WHEN `salary`>10000 THEN 'C级别'
ELSE 'D级别'
END AS 等级
FROM employees;