How to query column data in mysql

MySQL is a very widely used row database management system. It can handle large amounts of data and provides complete query and management tools. Querying column data is a very common and basic operation. In MySQL, querying column data requires the use of the SELECT statement. Let's take a look at it together.

1. Basic syntax

The SELECT statement is the basic statement used to query data in MySQL. It can query the data of the entire table or only certain columns of data. The basic syntax for querying column data is as follows:

SELECT column_name1,column_name2,...
FROM table_name

Among them, column_name1,column_name2,...represent the column names to be queried, you can Separate by commas to query multiple columns; table_name represents the name of the table where data needs to be queried.

2. Example

Below we use an example to demonstrate how to use the SELECT statement to query column data.

Suppose there is a students table, which contains the following information:

##张三男201##2##3王五男2114赵六女223Qianqi##6Liu BaMale193SELECT name,class_idFROM students

id	name	gender	age	class_id
1
李四	女	19	2
				##5
男	20	2
				Now we need to query the names (name) and names of all students in the students table The class number (class_id), the query statement is as follows:

Execute the above statement, the result is as follows:

name

class_id张三1李四2##Qian Qi2Liu Ba3ALL and DISTINCT keywords

##王五	1
赵六	3
It can be seen from the above results that querying column data can get some of the data information we need, helping us better analyze and process the data.	3. Other syntax

In the SELECT statement, we can also use the ALL and DISTINCT keywords to query the results Make restrictions. The ALL keyword indicates querying all data (default value), and the DISTINCT keyword indicates querying unique data. For example:

SELECT ALL name

FROM students
1. SELECT DISTINCT gender

FROM students

The above two statements respectively represent querying student name (name) data and Student gender (gender) does not duplicate data.

AS keyword

In the SELECT statement, we can also use the AS keyword to alias the query column name. For example:

SELECT name AS name, class_id AS class number

FROM students
2. In the above statement, the characters after the AS keyword represent the column name alias of the query result. In the final query result The alias will be displayed instead of the original column name.

4. Summary

Querying column data is a basic and common operation in MySQL, which can help us obtain some useful information from the data. When querying column data, you need to use the SELECT statement and specify the column name and table name. At the same time, the ALL and DISTINCT keywords can be used to limit the query results, and the AS keyword can be used to alias column names.

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