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MySQL tutorial 71-where conditional query data

2022-07-25 15:57:00 【51CTO】

stay MySQL in , If you need to query data conditionally from the data table , have access to WHERE Keyword to specify query criteria .

Use WHERE The syntax format of the keyword is as follows ：

Query criteria can be ：

Query conditions with comparison operators and logical operators
belt BETWEEN AND Keyword query conditions
belt IS NULL Keyword query conditions
belt IN Keyword query conditions
belt LIKE Keyword query conditions

Single condition query statement

A single condition refers to WHERE There is only one query condition after the keyword .

example 1

stay tb_students_info The height queried in the data table is 170cm Student name of ,SQL Statement and run results are as follows .

       
       mysql> SELECT name,height FROM tb_students_info
       
    -> WHERE height=170;
       
+-------+--------+
       
| name  | height |
       
+-------+--------+
       
| Susan |    170 |
       
+-------+--------+
       
1 row in set (0.17
      
1.
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6.
7.
8.

You can see , Recorded in the query results height The value of the field is equal to 170. If you query according to the specified conditions , There are no records matching the query conditions in the data table , The system will prompt “Empty set(0.00sec)”.

example 2

stay tb_students_info The query age in the data table is less than 22 Student name of ,SQL Statement and run results are as follows .

       
       mysql> SELECT name,age FROM tb_students_info
       
    -> WHERE age<22;
       
+------+------+
       
| name | age  |
       
+------+------+
       
| John |   21 |
       
+------+------+
       
1 row in set (0.05
      
1.
2.
3.
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5.
6.
7.
8.

You can see , All records in the query results age The values of the fields are all less than 22 year , And greater than or equal to 22 The record of years old was not returned .

Multi conditional query statement

stay WHERE There can be multiple query conditions after keywords , This can make the query results more accurate . Use logical operators when multiple query conditions AND（&&）、OR（||） or XOR separate .

AND： When all query conditions are met , Will be found out .
OR： When the record meets any query condition , Will be found out .
XOR： Record that one of the conditions is met , And do not meet another condition , Will be found out .

example 3

stay tb_students_info Query in table age Greater than 21, also height Greater than or equal to 175 Student information ,SQL Statement and run results are as follows .

       
       mysql> SELECT name,age,height FROM tb_students_info 
       
    -> WHERE age>21 AND height>=175;
       
+--------+------+--------+
       
| name   | age  | height |
       
+--------+------+--------+
       
| Henry  |   23 |    185 |
       
| Jim    |   24 |    175 |
       
| Thomas |   22 |    178 |
       
+--------+------+--------+
       
3 rows in set (0.00
      
1.
2.
3.
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5.
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You can see , All records in the query results age All fields are greater than 21 And height Fields are greater than or equal to 175.

example 4

stay tb_students_info Query in table age Greater than 21, perhaps height Greater than or equal to 175 Student information ,SQL Statement and run results are as follows .

       
       mysql> SELECT name,age,height FROM tb_students_info 
       
    -> WHERE age>21 OR height>=175;
       
+--------+------+--------+
       
| name   | age  | height |
       
+--------+------+--------+
       
| Dany   |   25 |    160 |
       
| Green  |   23 |    158 |
       
| Henry  |   23 |    185 |
       
| Jane   |   22 |    162 |
       
| Jim    |   24 |    175 |
       
| Lily   |   22 |    165 |
       
| Susan  |   23 |    170 |
       
| Thomas |   22 |    178 |
       
| Tom    |   23 |    165 |
       
+--------+------+--------+
       
9 rows in set (0.00
      
1.
2.
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You can see , All records in the query results age All fields are greater than 21 perhaps height Fields are greater than or equal to 175.

example 5

stay tb_students_info Query in table age Greater than 21, also height Less than 175 Student information and age Less than 21, also height Greater than or equal to 175 Student information ,SQL Statement and run results are as follows .

       
       mysql> SELECT name,age,height FROM tb_students_info 
       
    -> WHERE age>21 XOR height>=175;
       
+-------+------+--------+
       
| name  | age  | height |
       
+-------+------+--------+
       
| Dany  |   25 |    160 |
       
| Green |   23 |    158 |
       
| Jane  |   22 |    162 |
       
| Lily  |   22 |    165 |
       
| Susan |   23 |    170 |
       
| Tom   |   23 |    165 |
       
+-------+------+--------+
       
7 rows in set (0.00
      
1.
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3.
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5.
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You can see , All records in the query results age All fields are greater than 21 And height Fields are less than 175.tb_students_info Not in the data table age The field is less than 21 And height Field is greater than or equal to 175 The record of .

OR、AND and XOR Can be used together , But pay attention to the priority of operators when using

The more query conditions , The fewer records will be found . because , The more conditions you set , There are more restrictions on query statements , Fewer records can meet all the conditions . In order to make the queried records exactly what you want , Can be in WHERE Statement to set the query conditions more specifically .

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