基礎函數

upper：數據轉大寫

select upper(dname) from dept

lower：數據轉小寫

SELECT LOWER('ABC') from dept

length：數據的長度

SELECT LENGTH(dname) FROM dept

?substr：截取

SELECT dname,SUBSTR(dname,1,3) FROM dept

concat：拼接

SELECT dname,CONCAT(dname,"hello") FROM dept

replace：替換，把a替換成666

SELECT dname,REPLACE(dname,"a","666") FROM dept

ifnull：如果comm是null,用0替換

SELECT comm,IFNULL(comm,0) FROM emp

round：四舍五入并取整

SELECT ROUND(comm) FROM emp

ceil：向上取整

SELECT CEIL(comm) FROM emp

?floor：向下取整

SELECT FLOOR(comm) FROM emp

日期數據的精確處理

now（）：獲取當前時間

SELECT NOW()

獲取年、月、日、時、分、秒

SELECT YEAR(NOW()) //獲取當前時間的年份 SELECT YEAR('2000-1-1') //獲取括號中時間的年份 SELECT MONTH(NOW()) SELECT DAY(NOW()) SELECT HOUR(NOW()) SELECT MINUTE(NOW()) SELECT SECOND(NOW())

轉義字符

SELECT 'xi\'an'

條件查詢

distinct：去除重復的記錄行

SELECT DISTINCT loc FROM dept

where：不能使用列別名

SELECT * FROM dept WHERE deptno=2

like：模糊查詢

select * from emp where ename like 'a%' //以a開頭的select * from emp where ename like '%a' //以a結束的select * from emp where ename like '%a%' //中間包含a的

null

SELECT * FROM emp WHERE comm IS NULL SELECT * FROM emp WHERE comm IS NOT NULL

between....and.......

SELECT * FROM emp WHERE sal BETWEEN 5000 AND 10000

order by：默認是asc升序，加desc是降序

SELECT * FROM emp ORDER BY YEAR(hiredate) SELECT * FROM emp ORDER BY sal DESC

limit：分頁

SELECT * FROM emp LIMIT 2 SELECT * FROM emp LIMIT 0,3

聚合函數

count：計算個數

SELECT COUNT(*) FROM emp

max/min

SELECT MAX(sal) FROM emp SELECT MIN(sal) FROM emp

sum/avg

SELECT AVG(sal) FROM emp SELECT SUM(sal) FROM emp

group by：分組

SELECT job,COUNT(*)AS "人員" FROM emp GROUP BY job

having：分組后的過濾

SELECT COUNT(*),YEAR(hiredate) b FROM emp GROUP BY b HAVING b>2015

?各個關鍵字的順序:

• select * from 表名 where 普通字段條件 group by 分組字段名 having 聚合函數條件 order by 排序字段名 desc limit 跳過條數,請求條數;

總結

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