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MySQL tutorial 68-as setting alias

2022-07-25 15:57:00 【51CTO】

For the convenience of inquiry , MySQL Provides AS Keyword to specify aliases for tables and fields . This section mainly explains how to specify an alias for tables and fields .

Specify an alias for the table

When the table name is very long or some special queries are executed , For ease of operation , You can specify an alias for the table , Use this alias instead of the original name of the table .

The basic syntax format for specifying aliases for tables is ：

       
       < Table name > [AS] < Alias >
      
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The meanings of the clauses are as follows ：

< Table name >： The name of the data table stored in the database .
< Alias >： The new name of the table specified when querying .
AS Keywords can be omitted , After omitting, you need to separate the table name and alias with spaces .

Be careful ： The alias of a table cannot have the same name as other tables in this database . The alias of a field cannot have the same name as other fields in the table . Field aliases cannot be used in conditional expressions , Otherwise “ERROR 1054 (42S22): Unknown column” Such error message .

example 1

The following is tb_students_info Table specifying aliases stu,SQL Statement and run results are as follows .

       
       mysql> SELECT stu.name,stu.height FROM tb_students_info AS stu;
       
+--------+--------+
       
| name   | height |
       
+--------+--------+
       
| Dany   |    160 |
       
| Green  |    158 |
       
| Henry  |    185 |
       
| Jane   |    162 |
       
| Jim    |    175 |
       
| John   |    172 |
       
| Lily   |    165 |
       
| Susan  |    170 |
       
| Thomas |    178 |
       
| Tom    |    165 |
       
+--------+--------+
       
10 rows in set (0.04
      
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Specify an alias for the field

In the use of SELECT Statement to query data ,MySQL It will show each SELECT Specify the output fields later . Sometimes in order to display the results more intuitively , We can specify an alias for the field .

The basic syntax format for specifying an alias for a field is ：

       
       < Field name > [AS] < Alias >
      
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among , The grammatical meaning of each clause is as follows ：

< Field name >： The name defined for the field in the data table .
< Field alias >： Field new name .
AS Keywords can be omitted , If omitted, you need to separate the field name and alias with spaces .

example 2

Inquire about tb_students_info surface , by name Specify alias student_name, by age Specify alias student_age,SQL Statement and run results are as follows .

       
       mysql> SELECT name AS student_name, age AS student_age FROM tb_students_info;
       
+--------------+-------------+
       
| student_name | student_age |
       
+--------------+-------------+
       
| Dany         |          25 |
       
| Green        |          23 |
       
| Henry        |          23 |
       
| Jane         |          22 |
       
| Jim          |          24 |
       
| John         |          21 |
       
| Lily         |          22 |
       
| Susan        |          23 |
       
| Thomas       |          22 |
       
| Tom          |          23 |
       
+--------------+-------------+
       
10 rows in set (0.00
      
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        Be careful ： Table aliases are used only when executing queries , Does not show in the returned results . After the field defines the alias , Will return to the client to display , The field displayed as the alias of .
      
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