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MySQL tutorial 66 data table query statement

2022-07-25 15:56:00 【51CTO】

stay MySQL in , have access to SELECT Statement to query data . Querying data refers to retrieving data from the database according to requirements , Use different query methods to obtain different data , Is the most frequently used 、 The most important operation .

SELECT The syntax of is as follows ：

MySQL course 66- Data table query statement _ data

MySQL course 66- Data table query statement _ data _02

       
       SELECT
       
{* | < Field column name >}
       
[
       
FROM < surface  1>, < surface  2>…
       
[WHERE < expression >
       
[GROUP BY <group by definition>
       
[HAVING <expression> [{<operator> <expression>}…]]
       
[ORDER BY <order by definition>]
       
[LIMIT[<offset>,] <row count>]
       
]
      
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among , The meanings of each clause are as follows ：

{*|< Field column name >} List of fields containing asterisk wildcards , Indicates the name of the field to be queried .
< surface 1>,< surface 2>…, surface 1 And table 2 Indicates the source of query data , It can be one or more .
WHERE < expression > Optional. , If you select this item , The query data must meet the query condition .
GROUP BY< Field >, This clause tells MySQL How to display the queried data , And group according to the specified fields .
[ORDER BY< Field >], This clause tells MySQL In what order are the queried data displayed , You can sort in ascending order （ASC） And descending （DESC）, By default, it is in ascending order .
[LIMIT[<offset>,]<row count>], This clause tells MySQL Display the number of queried data each time .

Here are some simple SELECT sentence , About WHERE、GROUP BY、ORDER BY and LIMIT And so on , I'll explain them one by one later .

Query all fields in the table

Query all fields refers to the data of all fields in the query table .MySQL The following are provided 2 There are three ways to query all fields in the table .

Use “*” Wildcard Queries all fields
List all the fields of the table

1） Use “*” All fields of the query table

SELECT have access to “*” Look up the data of all fields in the table , The syntax is as follows ：

       
       SELECT * FROM
      
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Use “*” When inquiring , It can only be arranged in the order of the fields in the data table , You cannot change the order of fields .

example 1

from tb_students_info Query the data of all fields in the table ,SQL The statement and run result are as follows .

       
       mysql> use test_db;
       
Database changed
       
mysql> SELECT * FROM tb_students_info;
       
+----+--------+---------+------+------+--------+------------+
       
| id | name   | dept_id | age  | sex  | height | login_date |
       
+----+--------+---------+------+------+--------+------------+
       
|  1 | Dany   |       1 |   25 | F    |    160 | 2015-09-10 |
       
|  2 | Green  |       3 |   23 | F    |    158 | 2016-10-22 |
       
|  3 | Henry  |       2 |   23 | M    |    185 | 2015-05-31 |
       
|  4 | Jane   |       1 |   22 | F    |    162 | 2016-12-20 |
       
|  5 | Jim    |       1 |   24 | M    |    175 | 2016-01-15 |
       
|  6 | John   |       2 |   21 | M    |    172 | 2015-11-11 |
       
|  7 | Lily   |       6 |   22 | F    |    165 | 2016-02-26 |
       
|  8 | Susan  |       4 |   23 | F    |    170 | 2015-10-01 |
       
|  9 | Thomas |       3 |   22 | M    |    178 | 2016-06-07 |
       
| 10 | Tom    |       4 |   23 | M    |    165 | 2016-08-05 |
       
+----+--------+---------+------+------+--------+------------+
       
10 rows in set (0.26
      
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Results show , Use “*” When a wildcard is used , All columns will be returned , The data columns are displayed in the order in which the table was created .

       
        Be careful ： In general , Unless you need to use all the field data in the table , Otherwise, it is best not to use wildcards “*”. Although using wildcards can save time in entering query statements , However, getting unwanted column data often reduces the efficiency of queries and applications used . Use “*” The advantage is , When you don't know the name of the required column , Can pass “*” To get them .
      
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2） List all the fields of the table

SELECT The field name after the keyword is the field to be searched , Therefore, the names of all fields in the table can be followed by SELECT After keyword . If you forget the field name , have access to DESC Command to view the structure of the table .

Sometimes , Because there are many fields in the table , You may not remember the names of all fields , Therefore, this method is very inconvenient , Not recommended .

example 2

Inquire about tb_students_info All the data in the table ,SQL The statement can also be written as follows ：

       
       SELECT id,name,dept_id,age,sex,height,login_date FROM
      
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Running results and examples 1 identical .

This query method is more flexible , If you need to change the order of field display , Just adjust SELECT The list order of the fields after the keyword is ok .

Although the way to list all the fields of the table is flexible , However, when querying all fields, we usually use “*” wildcard . Use “*” This is the easy way , Especially when there are many fields in the table , The advantages of this method are more obvious . Of course , If you need to change the order of field display , You can choose to list all fields of the table .

The fields specified in the query table

The syntax format of a field in the query table is ：

       
       SELECT <  Name  > FROM <  Table name  >;
      
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example 3

Inquire about tb_students_info In the table name List the names of all students ,SQL The statement and run result are as follows .

       
       mysql> SELECT name FROM tb_students_info;
       
+--------+
       
| name   |
       
+--------+
       
| Dany   |
       
| Green  |
       
| Henry  |
       
| Jane   |
       
| Jim    |
       
| John   |
       
| Lily   |
       
| Susan  |
       
| Thomas |
       
| Tom    |
       
+--------+
       
10 rows in set (0.00
      
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The output shows tb_students_info In the table name All data under field .

Use SELECT Declare that you can get data under multiple fields , Just in the keyword SELECT Specify the name of the field to be found later , Use commas between different field names “,” Separate , There is no need to add a comma after the last field , The syntax is as follows ：

       
       SELECT < Field name 1>,< Field name 2>,…,< Field name n> FROM < Table name >;
      
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example 4

from tb_students_info Table acquisition id、name and height The three column ,SQL The statement and run result are as follows .

       
       mysql> SELECT id,name,height
       
    -> FROM tb_students_info;
       
+----+--------+--------+
       
| id | name   | height |
       
+----+--------+--------+
       
|  1 | Dany   |    160 |
       
|  2 | Green  |    158 |
       
|  3 | Henry  |    185 |
       
|  4 | Jane   |    162 |
       
|  5 | Jim    |    175 |
       
|  6 | John   |    172 |
       
|  7 | Lily   |    165 |
       
|  8 | Susan  |    170 |
       
|  9 | Thomas |    178 |
       
| 10 | Tom    |    165 |
       
+----+--------+--------+
       
10 rows in set (0.00
      
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