Tutorial on principles and applications of database system (042) -- MySQL query (4): using wildcards to construct query conditions

Database system principle and Application Tutorial （042）—— MySQL Inquire about （ Four ）： Use wildcards to construct query conditions

When querying data , Wildcards can replace one of the specified positions （ Underline ：_） Or more characters （ Percent sign ：%）.

One 、MySQL The wildcard

MySQL You can use wildcards as shown below ：

wildcard	describe
Percent sign （ % ）	Expressed in percent sign （%） The position can have any character , The content of characters is unlimited
Underline （ _ ）	Indicates in the underline （ _ ） The position has only one character , The content of characters is unlimited

explain ：

（1） If wildcards are used in the condition , The comparison operator must use LIKE, They can't be used =、>、< Equal operator . If the comparison operator is not used LIKE, be MySQL Think the percent sign and underscore are ordinary characters , Not wildcards .

（2） If wildcards are used in the condition , When querying, if the wildcard is placed on the left , Index will not be used .

Two 、 wildcard ： Percent sign （ % ）

When querying, it means that there can be any （0 - n individual ） character , The content of characters is unlimited .

for example ：

（1） Check the last name 【 Liu 】 Student information .

mysql> select * from student where s_name like ' Liu %';
+-------+-----------+---------------------+-------------+-----------+
| s_id  | s_name    | birth               | phone       | addr      |
+-------+-----------+---------------------+-------------+-----------+
| S2012 |  Liu Xiaoqing     | 1999-10-11 00:00:00 | 13603732255 |  Xinxiang City     |
| S2014 |  Liu Yan       | 1998-06-24 00:00:00 | 13623735335 |  Zhengzhou city     |
| S2015 |  Liu Yan       | 1999-07-06 00:00:00 | 13813735225 |  Xinyang City     |
| S2016 |  Liu Ruofei     | 2000-08-31 00:00:00 | 13683735533 |  Kaifeng City     |
+-------+-----------+---------------------+-------------+-----------+
4 rows in set (0.00 sec)

--  This query uses an index  idx_name
mysql> explain select * from student where s_name like ' Liu %'\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: student
   partitions: NULL
         type: range
possible_keys: idx_name
          key: idx_name
      key_len: 61
          ref: NULL
         rows: 4
     filtered: 100.00
        Extra: Using index condition
1 row in set, 1 warning (0.00 sec)

--  Wildcard usage  =, No query results , Unless the student's name is 【 Liu %】 Will be queried .
mysql> select * from student where s_name = ' Liu %';
Empty set (0.00 sec)

（2） The last word of the query is 【 Just. 】 Student information .

mysql> select * from student where s_name like '% Just. ';
+-------+-----------+---------------------+-------------+-----------+
| s_id  | s_name    | birth               | phone       | addr      |
+-------+-----------+---------------------+-------------+-----------+
| S2011 |  Zhang Xiaogang     | 1999-12-03 00:00:00 | 13163735775 |  Xinyang City     |
+-------+-----------+---------------------+-------------+-----------+
1 row in set (0.00 sec)

--  This query will not use indexes 
mysql> explain select * from student where s_name like '% Just. '\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: student
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 14
     filtered: 11.11
        Extra: Using where
1 row in set, 1 warning (0.00 sec)

（3） The query name contains 【 Hua 】 The student information of the word .

mysql> select * from student where s_name like '% Hua %';
+-------+-----------+---------------------+-------------+-----------+
| s_id  | s_name    | birth               | phone       | addr      |
+-------+-----------+---------------------+-------------+-----------+
| S2022 |  Zhou Huajian     | 1999-05-25 00:00:00 | 13243735578 |  Zhengzhou city     |
| S2025 |  Zhou Jianhua     | 2000-08-22 00:00:00 | 13788736655 |  Kaifeng City     |
+-------+-----------+---------------------+-------------+-----------+
2 rows in set (0.00 sec)

--  This query will not use indexes 
mysql> explain select * from student where s_name like '% Hua %'\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: student
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 14
     filtered: 11.11
        Extra: Using where
1 row in set, 1 warning (0.00 sec)

3、 ... and 、 wildcard ： Underline （ _ ）

When querying, it means that there is only one character in this position , The content of characters is unlimited . It has to be with LIKE Operators .

for example ：

（1） The second word in the query name is 【 Hua 】 Information about our students .

mysql> select * from student where s_name like '_ Hua %';
+-------+-----------+---------------------+-------------+-----------+
| s_id  | s_name    | birth               | phone       | addr      |
+-------+-----------+---------------------+-------------+-----------+
| S2022 |  Zhou Huajian     | 1999-05-25 00:00:00 | 13243735578 |  Zhengzhou city     |
+-------+-----------+---------------------+-------------+-----------+
1 row in set (0.00 sec)

--  This query will not use indexes 
mysql> explain select * from student where s_name like '_ Hua %'\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: student
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 14
     filtered: 11.11
        Extra: Using where
1 row in set, 1 warning (0.00 sec)

（2） Check the last name 【 Liu 】 And name a total of two words of student information .

mysql> select * from student where s_name like ' Liu _';
+-------+--------+---------------------+-------------+-----------+
| s_id  | s_name | birth               | phone       | addr      |
+-------+--------+---------------------+-------------+-----------+
| S2015 |  Liu Yan    | 1999-07-06 00:00:00 | 13813735225 |  Xinyang City     |
| S2014 |  Liu Yan    | 1998-06-24 00:00:00 | 13623735335 |  Zhengzhou city     |
+-------+--------+---------------------+-------------+-----------+
2 rows in set (0.00 sec)

--  This query uses an index  idx_name
mysql> explain select * from student where s_name like ' Liu _'\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: student
   partitions: NULL
         type: range
possible_keys: idx_name
          key: idx_name
      key_len: 61
          ref: NULL
         rows: 4
     filtered: 100.00
        Extra: Using index condition
1 row in set, 1 warning (0.00 sec)

（3） Inquire about phone The fourth to seventh numbers are 【2373】 Student information

--  Total at the beginning  3  Underscores 
mysql> select * from student where phone like '___2373%';
+-------+--------+---------------------+-------------+-----------+
| s_id  | s_name | birth               | phone       | addr      |
+-------+--------+---------------------+-------------+-----------+
| S2014 |  Liu Yan    | 1998-06-24 00:00:00 | 13623735335 |  Zhengzhou city     |
+-------+--------+---------------------+-------------+-----------+
1 row in set (0.00 sec)