MySQL之單表查詢
阿新 • • 發佈:2018-03-03
字符串 所有 order 多列 成員 off ati 表達式 opera 5.執行select
一、單表查詢的語法
SELECT 字段1,字段2...
FROM 表名
WHERE 條件
GROUP BY field
HAVING 篩選
ORDER BY field
LIMIT 限制條數
二、關鍵字的執行順序
1.找到表:from
2.拿著where指定的約束條件,去文件/表中取出一條條記錄
3.將取出的一條條記錄進行分組group by,如果沒有group by,則整體作為一組
4.將分組的結果進行having過濾
5.執行select
6.去重
7.將結果按條件排序:order by
8.限制結果的顯示條數
SELECT 字段1,字段2... ⑤
FROM 表名 ①
WHERE 條件 ②
GROUP BY field ③
HAVING 篩選 ④
ORDER BY field ⑥
LIMIT 限制條數 ⑦
三、簡單查詢
#簡單查詢 SELECT id,name,sex,age,hire_date,post,post_comment,salary,office,depart_id FROM employee; SELECT * FROM employee; SELECT name,salary FROM employee; #避免重復DISTINCT SELECT DISTINCT post FROM employee; #通過四則運算查詢 SELECT name, salary*12 FROM employee; SELECT name, salary*12 AS Annual_salary FROM employee; SELECT name, salary*12 Annual_salary FROM employee; #定義顯示格式 CONCAT() 函數用於連接字符串 SELECT CONCAT(‘姓名: ‘,name,‘ 年薪: ‘, salary*12) AS Annual_salary FROM employee; CONCAT_WS() 第一個參數為分隔符 SELECT CONCAT_WS(‘:‘,name,salary*12) AS Annual_salary FROM employee; 結合CASE語句: SELECT ( CASE WHEN NAME = ‘egon‘ THEN NAME WHEN NAME = ‘alex‘ THEN CONCAT(name,‘_BIGSB‘) ELSE concat(NAME, ‘SB‘) END ) as new_name FROM emp;
四、where約束
where字句中可以使用:
1. 比較運算符:> < >= <= <> !=
2. between 80 and 100 值在10到20之間
3. in(80,90,100) 值是10或20或30
4. like ‘egon%‘ pattern可以是%或_, %表示任意多字符 _表示一個字符
5. 邏輯運算符:在多個條件直接可以使用邏輯運算符 and or not
#1:單條件查詢 SELECT name FROM employee WHERE post=‘sale‘; #2:多條件查詢 SELECT name,salary FROM employee WHERE post=‘teacher‘ AND salary>10000; #3:關鍵字BETWEEN AND SELECT name,salary FROM employee WHERE salary BETWEEN 10000 AND 20000; SELECT name,salary FROM employee WHERE salary NOT BETWEEN 10000 AND 20000; #4:關鍵字IS NULL(判斷某個字段是否為NULL不能用等號,需要用IS) SELECT name,post_comment FROM employee WHERE post_comment IS NULL; SELECT name,post_comment FROM employee WHERE post_comment IS NOT NULL; SELECT name,post_comment FROM employee WHERE post_comment=‘‘; 註意‘‘是空字符串,不是null ps: 執行 update employee set post_comment=‘‘ where id=2; 再用上條查看,就會有結果了 #5:關鍵字IN集合查詢 SELECT name,salary FROM employee WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ; SELECT name,salary FROM employee WHERE salary IN (3000,3500,4000,9000) ; SELECT name,salary FROM employee WHERE salary NOT IN (3000,3500,4000,9000) ; #6:關鍵字LIKE模糊查詢 通配符’%’ SELECT * FROM employee WHERE name LIKE ‘eg%‘; 通配符’_’ SELECT * FROM employee WHERE name LIKE ‘al__‘;
五 分組查詢:GROUP BY
1 什麽是分組?為什麽要分組?
#1、首先明確一點:分組發生在where之後,即分組是基於where之後得到的記錄而進行的 #2、分組指的是:將所有記錄按照某個相同字段進行歸類,比如針對員工信息表的職位分組,或者按照性別進行分組等 #3、為何要分組呢? 取每個部門的最高工資 取每個部門的員工數 取男人數和女人數 小竅門:‘每’這個字後面的字段,就是我們分組的依據 #4、大前提: 可以按照任意字段分組,但是分組完畢後,比如group by post,只能查看post字段,如果想查看組內信息,需要借助於聚合函數
2 GROUP BY
單獨使用GROUP BY關鍵字分組 SELECT post FROM employee GROUP BY post; 註意:我們按照post字段分組,那麽select查詢的字段只能是post,想要獲取組內的其他相關信息,需要借助函數 GROUP BY關鍵字和GROUP_CONCAT()函數一起使用 SELECT post,GROUP_CONCAT(name) FROM employee GROUP BY post;#按照崗位分組,並查看組內成員名 SELECT post,GROUP_CONCAT(name) as emp_members FROM employee GROUP BY post; GROUP BY與聚合函數一起使用 select post,count(id) as count from employee group by post;#按照崗位分組,並查看每個組有多少人
強調:
如果我們用unique的字段作為分組的依據,則每一條記錄自成一組,這種分組沒有意義
多條記錄之間的某個字段值相同,該字段通常用來作為分組的依據
3 聚合函數
#強調:聚合函數聚合的是組的內容,若是沒有分組,則默認一組 示例: SELECT COUNT(*) FROM employee; SELECT COUNT(*) FROM employee WHERE depart_id=1; SELECT MAX(salary) FROM employee; SELECT MIN(salary) FROM employee; SELECT AVG(salary) FROM employee; SELECT SUM(salary) FROM employee; SELECT SUM(salary) FROM employee WHERE depart_id=3;
4 HAVING過濾
HAVING與WHERE不一樣的地方在於!!!!!!
#!!!執行優先級從高到低:where > group by > having #1. Where 發生在分組group by之前,因而Where中可以有任意字段,但是絕對不能使用聚合函數。 #2. Having發生在分組group by之後,因而Having中可以使用分組的字段,無法直接取到其他字段,可以使用聚合函數
mysql> select @@sql_mode; +--------------------+ | @@sql_mode | +--------------------+ | ONLY_FULL_GROUP_BY | +--------------------+ 1 row in set (0.00 sec) mysql> select * from emp where salary > 100000; +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+ | 2 | alex | male | 78 | 2015-03-02 | teacher | NULL | 1000000.31 | 401 | 1 | +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+ 1 row in set (0.00 sec) mysql> select * from emp having salary > 100000; ERROR 1463 (42000): Non-grouping field ‘salary‘ is used in HAVING clause mysql> select post,group_concat(name) from emp group by post having salary > 10000;#錯誤,分組後無法直接取到salary字段 ERROR 1054 (42S22): Unknown column ‘salary‘ in ‘having clause‘ mysql> select post,group_concat(name) from emp group by post having avg(salary) > 10000; +-----------+-------------------------------------------------------+ | post | group_concat(name) | +-----------+-------------------------------------------------------+ | operation | 程咬鐵,程咬銅,程咬銀,程咬金,張野 | | teacher | 成龍,jinxin,jingliyang,liwenzhou,yuanhao,wupeiqi,alex | +-----------+-------------------------------------------------------+ 2 rows in set (0.00 sec)驗證
六 查詢排序:ORDER BY
按單列排序 SELECT * FROM employee ORDER BY salary; SELECT * FROM employee ORDER BY salary ASC; SELECT * FROM employee ORDER BY salary DESC; 按多列排序:先按照age排序,如果年紀相同,則按照薪資排序 SELECT * from employee ORDER BY age, salary DESC;
七 限制查詢的記錄數:LIMIT
示例: SELECT * FROM employee ORDER BY salary DESC LIMIT 3; #默認初始位置為0 SELECT * FROM employee ORDER BY salary DESC LIMIT 0,5; #從第0開始,即先查詢出第一條,然後包含這一條在內往後查5條 SELECT * FROM employee ORDER BY salary DESC LIMIT 5,5; #從第5開始,即先查詢出第6條,然後包含這一條在內往後查5條
八 使用正則表達式查詢
MySQL中使用 REGEXP 操作符來進行正則表達式匹配。 模式 描述 ^ 匹配輸入字符串的開始位置。 $ 匹配輸入字符串的結束位置。 . 匹配任何字符(包括回車和新行) [...] 字符集合。匹配所包含的任意一個字符。例如, ‘[abc]‘ 可以匹配 "plain" 中的 ‘a‘。 [^...] 負值字符集合。匹配未包含的任意字符。例如, ‘[^abc]‘ 可以匹配 "plain" 中的‘p‘。 p1|p2|p3 匹配 p1 或 p2 或 p3。例如,‘z|food‘ 能匹配 "z" 或 "food"。‘(z|f)ood‘ 則匹配 "zood" 或 "food"。 # ^ 匹配 name 名稱 以 "e" 開頭的數據 select * from person where name REGEXP ‘^e‘; # $ 匹配 name 名稱 以 "n" 結尾的數據 select * from person where name REGEXP ‘n$‘; # . 匹配 name 名稱 第二位後包含"x"的人員 "."表示任意字符 select * from person where name REGEXP ‘.x‘; # [abci] 匹配 name 名稱中含有指定集合內容的人員 select * from person where name REGEXP ‘[abci]‘; # [^alex] 匹配 不符合集合中條件的內容 , ^表示取反 select * from person where name REGEXP ‘[^alex]‘; #註意1:^只有在[]內才是取反的意思,在別的地方都是表示開始處匹配 #註意2 : 簡單理解 name REGEXP ‘[^alex]‘ 等價於 name != ‘alex‘ # ‘a|x‘ 匹配 條件中的任意值 select * from person where name REGEXP ‘a|x‘; #查詢以w開頭以i結尾的數據 select * from person where name regexp ‘^w.*i$‘; #註意:^w 表示w開頭, .*表示中間可以有任意多個字符, i$表示以 i結尾
MySQL之單表查詢