| 運算符 | 說明 |
| = | 等于 |
| > 或者 != | 不等于 |
| > | 大于 |
| >= | 大于等于 |
| 小于 | |
| = | 小于等于 |
關系運算基本的語法格式如下:
select cname1,cname2,... from tname where cname operators cval
等于=
查詢出 列和后面的值嚴格相等的數據，非值類型的需要對后面值加上引號，值類型的不需要。
語法格式如下：
select cname1,cname2,... from tname where cname = cval;
mysql> select * from user2; +----+-------+-----+----------+-----+ | id | name | age | address | sex | +----+-------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | +----+-------+-----+----------+-----+ 3 rows in set mysql> select * from user2 where name='helen'; +----+-------+-----+----------+-----+ | id | name | age | address | sex | +----+-------+-----+----------+-----+ | 2 | helen | 20 | quanzhou | 0 | +----+-------+-----+----------+-----+ 1 row in set mysql> select * from user2 where age=21; +----+-------+-----+---------+-----+ | id | name | age | address | sex | +----+-------+-----+---------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 3 | sol | 21 | xiamen | 0 | +----+-------+-----+---------+-----+ 2 rows in set
不等于（>、!=）
不等于有兩種寫法，一種是>，另一種是!=，意思一樣，可隨意切換使用，但是 > 先于 != 出現，所以看很多以前的例子，> 出現頻率比較高，可移植性更強，推薦使用。
不等于的目的是查詢出與條件不符和結果，格式如下：
select cname1,cname2,... from tname where cname > cval; 或 select cname1,cname2,... from tname where cname != cval;
mysql> select * from user2; +----+-------+-----+----------+-----+ | id | name | age | address | sex | +----+-------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | +----+-------+-----+----------+-----+ 3 rows in set mysql> select * from user2 where age>20; +----+-------+-----+---------+-----+ | id | name | age | address | sex | +----+-------+-----+---------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 3 | sol | 21 | xiamen | 0 | +----+-------+-----+---------+-----+ 2 rows in set
大于小于（> ）
一般用于數值或者日期、時間類型的比較，格式如下：
select cname1,cname2,... from tname where cname > cval; select cname1,cname2,... from tname where cname cval; select cname1,cname2,... from tname where cname >= cval; select cname1,cname2,... from tname where cname = cval;
mysql> select * from user2 where age>20; +----+-------+-----+---------+-----+ | id | name | age | address | sex | +----+-------+-----+---------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 3 | sol | 21 | xiamen | 0 | +----+-------+-----+---------+-----+ 2 rows in set mysql> select * from user2 where age>=20; +----+-------+-----+----------+-----+ | id | name | age | address | sex | +----+-------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | +----+-------+-----+----------+-----+ 3 rows in set mysql> select * from user2 where age21; +----+-------+-----+----------+-----+ | id | name | age | address | sex | +----+-------+-----+----------+-----+ | 2 | helen | 20 | quanzhou | 0 | +----+-------+-----+----------+-----+ 1 row in set mysql> select * from user2 where age=21; +----+-------+-----+----------+-----+ | id | name | age | address | sex | +----+-------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | +----+-------+-----+----------+-----+ 3 rows in set
邏輯運算
| 運算符 | 說明 |
|---|---|
| AND | 多個條件都成立 |
| OR | 多個條件中滿足一個 |
| NOT | 對條件進行取非操作 |
AND（且）
當需要多個條件進行數據過濾的時候，使用這種方式，and的每個表達式都是要成立，過濾出來的數據就是用戶需要的。
下面過濾出年齡和性別兩個條件都成立的數據，語法格式如下：
select cname1,cname2,... from tname where cname1 operators cval1 and cname2 operators cval2
mysql> select * from user2; +----+-------+-----+----------+-----+ | id | name | age | address | sex | +----+-------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | | 4 | weng | 33 | guizhou | 1 | +----+-------+-----+----------+-----+ 4 rows in set mysql> select * from user2 where age >20 and sex=1; +----+-------+-----+---------+-----+ | id | name | age | address | sex | +----+-------+-----+---------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 4 | weng | 33 | guizhou | 1 | +----+-------+-----+---------+-----+ 2 rows in set
OR（或）
當多個條件中只要滿足一個條件即進行數據過濾。
下面條件過濾出年齡大于21歲和小于21歲的數據，語法格式如下：
select cname1,cname2,... from tname where cname1 operators cval1 or cname2 operators cval2
mysql> select * from user2; +----+-------+-----+----------+-----+ | id | name | age | address | sex | +----+-------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | | 4 | weng | 33 | guizhou | 1 | +----+-------+-----+----------+-----+ 4 rows in set mysql> select * from user2 where age>21 or age21; +----+-------+-----+----------+-----+ | id | name | age | address | sex | +----+-------+-----+----------+-----+ | 2 | helen | 20 | quanzhou | 0 | | 4 | weng | 33 | guizhou | 1 | +----+-------+-----+----------+-----+ 2 rows in set
NOT（取非）
對某個滿足的條件進行取反，過濾出來的數據就是用戶需要的。
下面過濾不屬于年齡大于20的數據，語法格式如下：
select cname1,cname2,... from tname where not(cname operators cval)
mysql> select * from user2; +----+-------+-----+----------+-----+ | id | name | age | address | sex | +----+-------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | | 4 | weng | 33 | guizhou | 1 | +----+-------+-----+----------+-----+ 4 rows in set mysql> select * from user2 where not(age>20); +----+-------+-----+----------+-----+ | id | name | age | address | sex | +----+-------+-----+----------+-----+ | 2 | helen | 20 | quanzhou | 0 | +----+-------+-----+----------+-----+ 1 row in set
模糊匹配
就像我們上面的那個用戶表信息表（包含名稱、年齡、地址、性別），當我們要查詢名稱為s開頭的用戶時，就可以用到 like 關鍵字了，他用以模糊匹配數據。
語法格式如下,pattern中可以包含通配符，有兩種。%：表示匹配任意一個或n個字符； _：表示匹配任意一個字符。
select cname1,cname2,... from tname where cname like pattern;
%的使用
mysql> select * from user2; +----+--------+-----+----------+-----+ | id | name | age | address | sex | +----+--------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | | 4 | weng | 33 | guizhou | 1 | | 5 | selina | 25 | taiwang | 0 | +----+--------+-----+----------+-----+ 5 rows in set mysql> select * from user2 where name like 's%'; +----+--------+-----+---------+-----+ | id | name | age | address | sex | +----+--------+-----+---------+-----+ | 3 | sol | 21 | xiamen | 0 | | 5 | selina | 25 | taiwang | 0 | +----+--------+-----+---------+-----+ 2 rows in set
_的使用
mysql> select * from user2; +----+--------+-----+----------+-----+ | id | name | age | address | sex | +----+--------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | | 4 | weng | 33 | guizhou | 1 | | 5 | selina | 25 | taiwang | 0 | +----+--------+-----+----------+-----+ 5 rows in set mysql> select * from user2 where name like 's_l'; +----+------+-----+---------+-----+ | id | name | age | address | sex | +----+------+-----+---------+-----+ | 3 | sol | 21 | xiamen | 0 | +----+------+-----+---------+-----+ 1 row in set
注意點
1、不要過度使用模糊匹配得通配符。如果其他操作符能達到相同的目的，應該使用其他操作符
2、對大體量的表進行模糊匹配的時候盡量不要以%開頭，比如 like '%username',這樣會執行掃表，效率較慢。盡量明確模糊查找的開頭部分，比如 like 'brand%'，會先定位到brand開頭的數據，效率高很多。
范圍值檢查
BETWEEN AND(區間查詢)
操作符 BETWEEN … AND 會選取介于兩個值之間的數據范圍，這些值可以是數值、文本或者日期，屬于一個閉區間查詢。
and 的左邊val1 和 右邊 val2 分別表示兩個臨界值，等同于數學公式[val1,val2] ，屬于這兩個區間的數據會被過濾出來（>=val1 和 =val2）,所以語法格式如下：
selec cname1,cname2,... from tname where cname between val1 and val2; 等同于 selec cname1,cname2,... from tname where cname >= val1 and cname = val2;
查詢年齡在[21,25]之間的數據：
mysql> select * from user2; +----+--------+-----+----------+-----+ | id | name | age | address | sex | +----+--------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | | 4 | weng | 33 | guizhou | 1 | | 5 | selina | 25 | taiwang | 0 | +----+--------+-----+----------+-----+ 5 rows in set mysql> select * from user2 where age between 21 and 25; +----+--------+-----+---------+-----+ | id | name | age | address | sex | +----+--------+-----+---------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 3 | sol | 21 | xiamen | 0 | | 5 | selina | 25 | taiwang | 0 | +----+--------+-----+---------+-----+ 3 rows in set mysql> select * from user2 where age >= 21 and age = 25; +----+--------+-----+---------+-----+ | id | name | age | address | sex | +----+--------+-----+---------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 3 | sol | 21 | xiamen | 0 | | 5 | selina | 25 | taiwang | 0 | +----+--------+-----+---------+-----+ 3 rows in set
IN（包含查詢）
按照上面得數據，如果我們想查出居住地位于福州和廈門得用戶數據，應該使用 IN操作符，因為 IN 操作符允許我們在 WHERE 子句中指定多個值，符合這些值中得某一項，既滿足條件返回數據。
語法格式如下，in 后面列表的值類型必須一致或兼容，且不支持通配符：
select cname1,cname2,... from tname where cname in (val1,val2,...);
mysql> select * from user2;
+----+--------+-----+----------+-----+
| id | name | age | address | sex |
+----+--------+-----+----------+-----+
| 1 | brand | 21 | fuzhou | 1 |
| 2 | helen | 20 | quanzhou | 0 |
| 3 | sol | 21 | xiamen | 0 |
| 4 | weng | 33 | guizhou | 1 |
| 5 | selina | 25 | taiwang | 0 |
+----+--------+-----+----------+-----+
5 rows in set
mysql> select * from user2 where address in('fuzhou','xiamen');
+----+-------+-----+---------+-----+
| id | name | age | address | sex |
+----+-------+-----+---------+-----+
| 1 | brand | 21 | fuzhou | 1 |
| 3 | sol | 21 | xiamen | 0 |
+----+-------+-----+---------+-----+
2 rows in set
NOT IN（對包含查詢取反）
我們上面已經學習過了not得用戶，對not后面執行得表達式進行取反得操作，測試下：
mysql> select * from user2;
+----+--------+-----+----------+-----+
| id | name | age | address | sex |
+----+--------+-----+----------+-----+
| 1 | brand | 21 | fuzhou | 1 |
| 2 | helen | 20 | quanzhou | 0 |
| 3 | sol | 21 | xiamen | 0 |
| 4 | weng | 33 | guizhou | 1 |
| 5 | selina | 25 | taiwang | 0 |
+----+--------+-----+----------+-----+
5 rows in set
mysql> select * from user2 where address not in('fuzhou','quanzhou','xiamen');
+----+--------+-----+---------+-----+
| id | name | age | address | sex |
+----+--------+-----+---------+-----+
| 4 | weng | 33 | guizhou | 1 |
| 5 | selina | 25 | taiwang | 0 |
+----+--------+-----+---------+-----+
2 rows in set
空值檢查
IS NULL/IS NOT NULL
判斷是否為空，語法格式如下，這邊注意的是，對值為null的數據，各種比較運算符、like、between and、in、not in查詢都不起作用，只有is null 能夠過濾出來。
select cname1,cname2,... from tname where cname is null; 或者 select cname1,cname2,... from tname where cname is not null;
mysql> select * from user2 where address is null; +----+--------+-----+---------+-----+ | id | name | age | address | sex | +----+--------+-----+---------+-----+ | 5 | selina | 25 | NULL | 0 | +----+--------+-----+---------+-----+ 1 row in set mysql> select * from user2 where address is not null; +----+-------+-----+----------+-----+ | id | name | age | address | sex | +----+-------+-----+----------+-----+ | 1 | brand | 21 | fuzhou | 1 | | 2 | helen | 20 | quanzhou | 0 | | 3 | sol | 21 | xiamen | 0 | | 4 | weng | 33 | guizhou | 1 | +----+-------+-----+----------+-----+ 4 rows in set
有一種關鍵字 =>，可以包含對null值得判斷，但是目前用的比較少了，有興趣可以去查查，這邊不贅述。
總結
1、like表達式中的%匹配一個到多個任意字符，_匹配一個任意字符
2、空值查詢需要使用IS NULL或者IS NOT NULL，其他查詢運算符對NULL值無效。即使%通配符可以匹配任何東西，也不能匹配值NULL的數據。
3、建議創建表的時候，表字段不設置空，給字段一個default 默認值。
4、MySQL支持使用NOT對IN 、BETWEEN 和EXISTS子句取反 。
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標簽：惠州 牡丹江 合肥 天津 阿里 呼和浩特 沈陽 公主嶺
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