MySQL全面瓦解之查詢的過濾條件詳解

阿新 • • 發佈：2020-11-12

概述

在實際的業務場景應用中，我們經常要根據業務條件獲取並篩選出我們的目標資料。這個過程我們稱之為資料查詢的過濾。而過濾過程使用的各種條件（比如日期時間、使用者、狀態）是我們獲取精準資料的必要步驟，

這樣才能得到我們期望的結果。所以本章我們來學習MySQL中查詢過濾條件的各種用法。

關係運算

關係運算就是where語句後跟上一個或者n個條件，滿足where後面條件的資料會被返回，反之不滿足的就會被過濾掉。operators指的是運算子，有如下幾種情況：

運算子說明

= 等於

<> 或者 != 不等於

> 大於

>= 大於等於

< 小於

<= 小於等於

關係運算基本的語法格式如下:

 select cname1,cname2,... from tname where cname operators cval

等於=

查詢出列和後面的值嚴格相等的資料，非值型別的需要對後面值加上引號，值型別的不需要。

語法格式如下：

select cname1,... 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

不等於（<>、!=）

不等於有兩種寫法，一種是<>，另一種是!=，意思一樣，可隨意切換使用，但是 <> 先於 != 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。

不等於的目的是查詢出與條件不符和結果，格式如下：

select cname1,... from tname where cname <> cval;
或
select cname1,... 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

大於小於（> <）

一般用於數值或者日期、時間型別的比較，格式如下：

select cname1,... from tname where cname > cval;

select cname1,... from tname where cname < cval;

select cname1,... from tname where cname >= cval;

select cname1,... 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 age<21;
+----+-------+-----+----------+-----+
| 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

運算子說明

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

運算子	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資料過濾的時候，使用這種方式，and的每個表示式都是要成立，過濾出來的資料就是使用者需要的。

下面過濾出年齡和性別兩個條件都成立的資料，語法格式如下：

select cname1,... 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資料過濾。

下面條件過濾出年齡大於21歲和小於21歲的資料，語法格式如下：

select cname1,... 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 age<21;
+----+-------+-----+----------+-----+
| 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，過濾出來的資料就是使用者需要的。

下面過濾不屬於年齡大於20的資料，語法格式如下：

select cname1,... 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使用者表資訊表（包含名稱、年齡、地址、性別），當我們要查詢名稱為s開頭的使用者時，就可以用到 like 關鍵字了，他用以模糊匹配資料。

語法格式如下,pattern中可以包含萬用字元，有兩種。%：表示匹配任意一個或n個字元； _：表示匹配任意一個字元。

select cname1,... 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萬用字元。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儘量不要以%開頭，比如 like '%username',這樣會執行掃表，效率較慢。儘量明確模糊查詢的開頭部分，比如 like 'brand%'，會先定位到brand開頭的資料，效率高很多。

範圍值檢查

BETWEEN AND(區間查詢)

操作符 BETWEEN … AND 會選取介於兩個值之間的資料範圍，這些值可以是數值、文字或者日期，屬於一個閉區間查詢。

and 的左邊val1 和右邊 val2 分別表示兩個臨界值，等同於數學公式[val1,val2] ，屬於這兩個區間的資料會被過濾出來（>=val1 和 <=val2）,所以語法格式如下：

 selec cname1,... from tname where cname between val1 and val2;
 等同於
 selec cname1,... from tname where cname >= val1 and cname <= val2;

查詢年齡在[21,25]之間的資料：

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）

按照上面得資料，如果我們想查出居住地位於福州和廈門得使用者資料，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，既滿足條件返回資料。

語法格式如下，in 後面列表的值型別必須一致或相容，且不支援萬用字元：

select cname1,... 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得使用者，對not後面執行得表示式進行取反得操作，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的資料，各種比較運算子、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,... from tname where cname is null;
 或者
 select cname1,... 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表示式中的%匹配一個到多個任意字元，_匹配一個任意字元

2、空值查詢需要使用IS NULL或者IS NOT NULL，其他查詢運算子對NULL值無效。即使%萬用字元可以匹配任何東西，也不能匹配值NULL的資料。

3、建議建立表的時候，表字段不設定空，給欄位一個default 預設值。

4、MySQL支援使用NOT對IN 、BETWEEN 和EXISTS子句取反。

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