mysql query data strip data

MySQL is one of the most commonly used open source relational database management systems. It adopts a client/server architecture and has the characteristics of high speed, ease of use and flexibility. In daily database management, querying data is one of the most basic operations. This article will introduce the method of querying data in MySQL, hoping to help everyone handle the database better.

1. Select the database and table to query

Before using MySQL to query data, you first need to select the database and table to query. You can connect to the MySQL server through the following command:

mysql -h 主机名 -u 用户名 -p

After entering the password, you can enter the MySQL environment. Next, select the database to be queried:

use 数据库名;

If the table to be queried is not in the database, you need to enter other databases or create a new table first.

2. Query the complete table data

The command to query the complete table data is very simple:

select * from 表名;

This command will return all the data in the table. It should be noted that the asterisk (*) indicates the column to be queried and can be replaced by a specific column name. If the table you are querying is larger, you may need to wait longer for results to be returned.

3. Query table data under specified conditions

In practical applications, we often need to query table data that meets specific conditions. At this time, you can use the where clause to limit the query scope. The format of the where clause is:

select 列1，列2 from 表名 where 条件;

Among them, column 1 and column 2 represent the columns to be queried, and can be replaced by an asterisk (*). Conditions are composed of operators and values. Commonly used operators include equal to (=), greater than (>), less than (<), not equal to (!=), etc. For example, to query the data in the student table that is older than 20 years old, you can use the following command:

select * from 学生 where 年龄 > 20;</p>
<p>It should be noted that the where clause can use keywords such as and, or, and like to combine operations. </p>
<ol start="4"><li>Sort query results</li></ol>
<p>Sort of query results is also one of the common requirements. Sorting can be done in ascending (ASC) or descending (DESC) order. For example, to sort the students table in ascending order of age, you can use the following command: </p>
<pre class="brush:php;toolbar:false">select * from 学生 order by 年龄 ASC;

This will return the student data sorted by age from small to large. If you want to sort by multiple columns, you can continue to add the corresponding column names and sorting methods after order by.

5. Statistical query results

Statistical query results can be used for data summary and analysis. Among them, it is very useful to understand statistical information such as average, maximum, and minimum values. Aggregation functions can be used at this time, including COUNT, SUM, AVG, MAX and MIN, etc. For example, to count the average age in the student table, you can use the following command:

select AVG(年龄) from 学生;

It should be noted that the aggregate function can only be used for numeric columns, and the query results will be summarized, so only Returns a value.

6. Group query results

Group query refers to grouping the results according to specific columns to analyze and collect statistics on the data. For example, to group by the average age of different classes in the student table, you can use the following command:

select 班级，AVG(年龄) from 学生 group by 班级;

This will return the average age data grouped by class. It should be noted that the group by clause must appear after the where clause and before the order by clause.

7. Result Set Limitations

If the query result set is very large, you may need to limit it to avoid the data becoming too large. MySQL provides two keywords to limit the result set, namely LIMIT and OFFSET. LIMIT is used to set the maximum number of rows returned for query results, and OFFSET is used to set the starting position of query results. For example, to find the first 10 pieces of data in the student table whose age is greater than 20 years old, you can use the following command:

select * from 学生 where 年龄 > 20 limit 0, 10;

This will return a total of 10 pieces of student data that meet the conditions starting from the 1st piece of data.

In short, MySQL query data is one of the most basic operations for database operations. Learning to master the method of MySQL query data is very helpful for daily database query operations, and it is also the basis for further in-depth use of MySQL. I wish everyone can successfully handle the database when using MySQL!

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