How to query row data in mysql

MySQL is a very popular relational database management system that supports various methods of retrieving data from tables, the most commonly used of which is querying row data. This article will introduce how to use MySQL to query row data, including basic queries, conditional queries and multi-table queries.

1. Basic query

In MySQL, the most basic row query statement is the SELECT statement. Its basic form is:

SELECT column1, column2, ... FROM table_name;

where column1, column2, etc. represent the column names to be retrieved, and table_name represents the table name to be retrieved.

For example, if we have a table named "students" that contains two columns of data: "name" and "age", then if we want to query the names and ages of all students, we can use the following statement:

SELECT name, age FROM students;

After running this statement, MySQL will return a table containing the names and ages of all students.

2. Conditional query

Basic query can retrieve all row data in the table, but a more common situation is to only retrieve row data that meets certain conditions. MySQL provides a variety of methods to implement this conditional retrieval, such as WHERE clause, LIKE clause and IN clause.

1. WHERE clause

The WHERE clause can be used to limit the conditions that the returned row data must meet. Its syntax is as follows:

SELECT column1, column2, ... FROM table_name WHERE condition;

where condition is some logical expressions describing row data restrictions, which can include comparison operators, Logical operators and functions, etc.

For example, if we only want to query students who are older than 20 years old, we can use the following statement:

SELECT name, age FROM students WHERE age > 20;

After running this statement, MySQL returns a table containing the names and ages of all students older than 20 years old.

2. LIKE clause

The LIKE clause can be used to match specific string patterns. Its syntax is as follows:

SELECT column1, column2, ... FROM table_name WHERE column_name LIKE pattern;

where pattern is a string containing wildcard characters. Wildcard characters can be used to match any character or A set of characters. The most commonly used wildcard characters are % and _, which respectively match any character and match a single character.

For example, if we only want to query students whose names start with "Li", we can use the following statement:

SELECT name, age FROM students WHERE name LIKE 'Li%';

After running this statement, MySQL will return a table containing the names and ages of all students whose names begin with "Li".

3. IN clause

The IN clause can be used to retrieve whether the value in a column belongs to a specified set of values. Its syntax is as follows:

SELECT column1, column2, ... FROM table_name WHERE column_name IN (value1, value2, ...);

where value1, value2, etc. represent the values ​​to be matched .

For example, if we only want to query students whose names are "Li" or "Zhang", we can use the following statement:

SELECT name, age FROM students WHERE name IN ('Li' , 'Zhang');

After running this statement, MySQL will return a table containing the names and ages of all students named "Li" or "Zhang".

3. Multi-table query

In practical applications, it is usually necessary to jointly retrieve data from multiple tables. MySQL provides a variety of methods to implement this multi-table query, including INNER JOIN, LEFT JOIN and RIGHT JOIN, etc.

1. INNER JOIN

INNER JOIN is one of the most commonly used joint query methods, which can simultaneously retrieve related data based on the association conditions between two tables. Its syntax is as follows:

SELECT column1, column2, ... FROM table1 INNER JOIN table2 ON condition;

where table1 and table2 represent the two tables to be connected, and condition is the connection condition. Usually some columns from both tables.

For example, if we have two tables "students" and "scores", the "students" table contains the students' names and ages, and the "scores" table contains the students' names and grades, then we want to query each For each student’s name, age and score, you can use the following statement:

SELECT students.name, students.age, scores.score FROM students INNER JOIN scores ON students.name = scores.name;

After running this statement, MySQL will return a table containing the name, age and grade of each student.

2. LEFT JOIN

LEFT JOIN is also a joint query method, which can return all row data in the left table and qualified row data in the right table. Its syntax is as follows:

SELECT column1, column2, ... FROM table1 LEFT JOIN table2 ON condition;

where table1 and table2 represent the two tables to be connected, and condition is the connection condition. Usually some columns from both tables.

For example, if we have two tables "students" and "scores" and want to query the names, ages and grades (if any) of all students, we can use the following statement:

SELECT students.name, students.age, scores.score FROM students LEFT JOIN scores ON students.name = scores.name;

After running this statement, MySQL will return a list containing the names, ages and scores of all students ( if any) form.

3. RIGHT JOIN

RIGHT JOIN is similar to LEFT JOIN, but returns all the row data in the right table and the left table data that meets the conditions. Its syntax is as follows:

SELECT column1, column2, ... FROM table1 RIGHT JOIN table2 ON condition;

where table1 and table2 represent the two tables to be connected, and condition is the connection condition. Usually some columns from both tables.

For example, if we have two tables "students" and "scores" and want to query all score records and the corresponding student's name and age (if any), we can use the following statement:

SELECT students.name, students.age, scores.score FROM students RIGHT JOIN scores ON students.name = scores.name;

After running this statement, MySQL will return a list containing all score records and corresponding students Form of name and age (if any).

Summary

This article introduces how to use MySQL to query row data, including basic queries, conditional queries and multi-table queries. Among them, basic query can retrieve all data in the table, conditional query can retrieve data according to specified conditions, and multi-table query can jointly retrieve data from multiple tables. Using these techniques can help us retrieve data faster and more accurately, and improve the efficiency and accuracy of data processing.

The above is the detailed content of How to query row data in mysql. For more information, please follow other related articles on the PHP Chinese website!