MySQL全面瓦解之查詢的過濾條件詳解
概述
在實(shí)際的業(yè)務(wù)場(chǎng)景應(yīng)用中，我們經(jīng)常要根據(jù)業(yè)務(wù)條件獲取并篩選出我們的目標(biāo)數(shù)據(jù)。這個(gè)過程我們稱之為數(shù)據(jù)查詢的過濾。而過濾過程使用的各種條件（比如日期時(shí)間、用戶、狀態(tài)）是我們獲取精準(zhǔn)數(shù)據(jù)的必要步驟，
這樣才能得到我們期望的結(jié)果。所以本章我們來學(xué)習(xí)MySQL中查詢過濾條件的各種用法。
關(guān)系運(yùn)算
關(guān)系運(yùn)算就是where語句后跟上一個(gè)或者n個(gè)條件，滿足where后面條件的數(shù)據(jù)會(huì)被返回，反之不滿足的就會(huì)被過濾掉。operators指的是運(yùn)算符 ，有如下幾種情況：
| 運(yùn)算符 | 說明 |
| = | 等于 |
| <> 或者 != | 不等于 |
| > | 大于 |
| >= | 大于等于 |
| < | 小于 |
| <= | 小于等于 |
關(guān)系運(yùn)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(yán)格相等的數(shù)據(jù)，非值類型的需要對(duì)后面值加上引號(hào)，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(xiàn)，所以看很多以前的例子，<> 出現(xiàn)頻率比較高，可移植性更強(qiáng)，推薦使用。
不等于的目的是查詢出與條件不符和結(jié)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(shù)值或者日期、時(shí)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 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(yùn)算
| 運(yùn)算符 | 說明 |
|---|---|
| AND | 多個(gè)條件都成立 |
| OR | 多個(gè)條件中滿足一個(gè) |
| NOT | 對(duì)條件進(jìn)行取非操作 |
AND（且）
當(dāng)需要多個(gè)條件進(jìn)行數(shù)據(jù)過濾的時(shí)候，使用這種方式，and的每個(gè)表達(dá)式都是要成立，過濾出來的數(shù)據(jù)就是用戶需要的。
下面過濾出年齡和性別兩個(gè)條件都成立的數(shù)據(jù)，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(dāng)多個(gè)條件中只要滿足一個(gè)條件即進(jìn)行數(shù)據(jù)過濾。
下面條件過濾出年齡大于21歲和小于21歲的數(shù)據(jù)，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 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(duì)某個(gè)滿足的條件進(jìn)行取反，過濾出來的數(shù)據(jù)就是用戶需要的。
下面過濾不屬于年齡大于20的數(shù)據(jù)，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(gè)用戶表信息表（包含名稱、年齡、地址、性別），當(dāng)我們要查詢名稱為s開頭的用戶時(shí)，就可以用到 like 關(guān)鍵字了，他用以模糊匹配數(shù)據(jù)。
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(gè)或n個(gè)字符； _：表示匹配任意一個(gè)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(diǎn)
1、不要過度使用模糊匹配得通配符。如果其他操作符能達(dá)到相同的目的，應(yīng)該使用其他操作符
2、對(duì)大體量的表進(jìn)行模糊匹配的時(shí)候盡量不要以%開頭，比如 like '%username',這樣會(huì)執(zhí)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(huì)先定位到brand開頭的數(shù)據(jù)，效率高很多。
范圍值檢查
BETWEEN AND(區(qū)間查詢)
操作符 BETWEEN … AND 會(huì)選取介于兩個(gè)值之間的數(shù)據(jù)范圍，這些值可以是數(shù)值、文本或者日期，屬于一個(gè)閉區(qū)間查詢。
and 的左邊val1 和 右邊 val2 分別表示兩個(gè)臨界值，等同于數(shù)學(xué)公式[val1,val2] ，屬于這兩個(gè)區(qū)間的數(shù)據(jù)會(huì)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(shù)據(jù)：
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(shù)據(jù)，如果我們想查出居住地位于福州和廈門得用戶數(shù)據(jù)，應(yīng)該使用 IN操作符，因?yàn)?IN 操作符允許我們?cè)?WHERE 子句中指定多個(gè)值，符合這些值中得某一項(xiàng)，既滿足條件返回?cái)?shù)據(jù)。
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(duì)包含查詢?nèi)》矗?/strong>
我們上面已經(jīng)學(xué)習(xí)過了not得用戶，對(duì)not后面執(zhí)行得表達(dá)式進(jìn)行取反得操作，測(cè)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(duì)值為null的數(shù)據(jù)，各種比較運(yùn)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(guān)鍵字 <=>，可以包含對(duì)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(jié)
1、like表達(dá)式中的%匹配一個(gè)到多個(gè)任意字符，_匹配一個(gè)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(yùn)算符對(duì)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(shù)據(jù)。
3、建議創(chuàng)建表的時(shí)候，表字段不設(shè)置空，給字段一個(gè)default 默認(rèn)值。
4、MySQL支持使用NOT對(duì)IN 、BETWEEN 和EXISTS子句取反 。
到此這篇關(guān)于MySQL全面瓦解之查詢的過濾條件的文章就介紹到這了,更多相關(guān)MySQL查詢的過濾條件內(nèi)容請(qǐng)搜索腳本之家以前的文章或繼續(xù)瀏覽下面的相關(guān)文章希望大家以后多多支持腳本之家！
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