SELECT查詢語句
---進行單條記錄、多條記錄、單表、多表、子查詢……
SELECT [ALL | DISTINCT | DISTINCTROW ] [HIGH_PRIORITY] [MAX_STATEMENT_TIME = N] [STRAIGHT_JOIN] [SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT] [SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS] select_expr [, select_expr ...] [FROM table_references [PARTITION partition_list] [WHERE where_condition] [GROUP BY {col_name | expr | position} [ASC | DESC], ... [WITH ROLLUP]] [HAVING where_condition] [ORDER BY {col_name | expr | position} [ASC | DESC], ...] [LIMIT {[offset,] row_count | row_count OFFSET offset}] [PROCEDURE procedure_name(argument_list)] [INTO OUTFILE 'file_name' [CHARACTER SET charset_name] export_options | INTO DUMPFILE 'file_name'
| INTO var_name [, var_name]] [FOR UPDATE | LOCK IN SHARE MODE]]
select書寫使用技巧：
①確認需要訪問數據來自哪幾張表
from來自某張表或者某幾張表
join添加某張表
on表連接條件
記住一點：每關聯一個表就需要加上對應的on條件（on條件就是主外鍵條件）
②通過where條件過濾數據
③確認需求里面是否有分組聚合的含義
分組：group by
聚合：聚合函數
聚合條件過濾：having
④是否需要排序
order by
1、查詢某張表所有數據
mysql> select * from temp; 解析：*代表所有列，temp代表表名，不帶條件就查詢所有數據
2、查詢指定列和條件的數據
mysql> select name,age from temp where age = 22; 解析：查詢name和age這兩列，age 等於22的數據。
3、對查詢的數據進行運算操作
mysql> select age+2,age/2,age-2,age*2 from temp where age-2 > 22; mysql> select PLAYRNO,AMOUNT,AMOUNT*6.5 “Ren Min Bi” from PENALTIES; 解析：查詢AMOUNT列數據乘6.5並改列名為Ren Min Bi 使用小括號可以改變運算的優先級
4、concat函數，字符串連接
mysql> select NAME,concat(TOWN,STREET,HOUSENO) “player Home Address” from PLAYERS; 解析：利用concat函數將TOWN,STREET,HOUSENO三列的字符串連接起來，別名列為player Home Address(可以在concat里加' '進行分隔) 注意：concat和null進行連接，會導致連接后的數據成為null mysql> select * from t1; +------+--------+--------+
| id | First | Last |
+------+--------+--------+
| 1 | zhang | jiacai |
| 2 | linghu | NULL |
+------+--------+--------+ mysql> select id,concat(First,' ',Last) from t1; +------+------------------------+
| id | concat(First,' ',Last) |
+------+------------------------+
| 1 | zhang jiacai |
| 2 | NULL |
+------+------------------------+
concat_ws函數，指定分隔符的字符串連接
mysql> select id,concat_ws(':',First,Last) "Full Name" from t1; +------+--------------+
| id | Full Name |
+------+--------------+
| 1 | zhang:jiacai |
| 2 | linghu |
+------+--------------+ 圓括號里的第一個位置用來指定字符串連接的分隔符
5、as 列別名(或省略，留空格)
mysql> select id as 'num' from t1; mysql> select id 'num',First from t1;
6、distinct關鍵字去掉重復數據
mysql> select * from t2; +------+
| num |
+------+
| 1 |
| 1 |
| 1 |
| 2 |
+------+ mysql> select distinct num from t2; +------+
| num |
+------+
| 1 |
| 2 |
+------+ distinct 多列：去重的是同行多列組合的重復數據 mysql> select distinct id, age from temp;
7、where條件查詢
where語句中的條件比較：大於>、大於等於>=、小於<、小於等於<=、等於=、不等於<> mysql> select * from tableName where a>2 or a>=3 or a<5 or a<=6 or a=7 or a<>0;
8、and 並且、or 或者、not非
mysql> select * from temp where age>20 and name=‘jack’; 解析：查詢name等於jack並且年齡大於20的 mysql> select * from tmep where name=‘jack’ or name=‘jackson’; 解析：查詢name是jack或是jackson的 mysql> select * from temp where not (age > 20); 解析：取小於等於20的數據 mysql> select * from temp where id not in(1, 2); 解析：查詢id數不是1，也不是2的
9、between v1 and v2：v1和v2之間
mysql> select * form temp where age between 20 and 25; 解析：查詢age在20和25之間的
10、in 查詢：多個條件 類似於or
mysql> select * from temp where id in (1, 2, 3); 解析：查詢id在括號中出現的數據
11、like 模糊查詢
%:替代0個或多個字符 _：替代一個字符 mysql> select * from temp where name like ‘j%’; 解析：查詢name以j開頭的(%通配所有) mysql> select * from temp where name like ‘%k%’; 解析：查詢name包含k的 escape轉義 mysql> select * from temp where name like ‘\_%’ escape ‘\’; 解析：指定\為轉義字符，上面的就可以查詢name中以“_”開頭的數據
12、is null、is not null
mysql> select * from temp where name is null; 解析：查詢為null的數據 mysql> select * from temp where name is not null; 解析：查詢不為null的數據
13、order by排序：desc降序、asc升序
mysql> select * from temp order by id; (默認asc升排序) mysql> select * from temp order by id desc; (指定降序排) 多列組合 mysql> select * from temp order by id, age;
14、limit子句：從結果集中選取最前面或最后面的幾行
通常和order by連用，放其后面
limit <獲取的行數> [OFFSET <跳過的行數>]
limit [<跳過的行數>,] <獲取的行數>
mysql> select playerno,name from players order by playerno asc limit 3,5; mysql> select playerno,name from players order by playerno asc limit 5 offset 3; 解析：跳過前面的3行，從第4行開始取，取5行
注意：MySQL5.7 doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery'
15、group by、having 分組聚合
select [聚合函數] 字段名 from 表名
[where 查詢條件]
[group by 字段名]
[having 過濾條件]
mysql> select salary,count(*) from salary_tab -> where salary>=2000
-> group by salary -> having count(*)>=0; +---------+----------+
| salary | count(*) |
+---------+----------+
| 2000.00 | 1 |
| 3000.00 | 1 |
+---------+----------+
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