[Summary] Commonly used query statements in PHP and how to use them

In web development, it is often necessary to query data from the database and present it to users in a specific format. PHP language is a commonly used Web programming language and one of the important tools for processing database queries. This article will introduce commonly used query statements in PHP and how to use them.

1. SELECT statement

The SELECT statement is used to select data from the database. The syntax is:

SELECT column_name(s) FROM table_name

Among them, column_name is the name of the column that needs to be queried, which can be one or more column names. Multiple column names are separated by commas; table_name is the name of the table that needs to be queried.

For example, to query all the data in the students table, you can use the following statement:

SELECT * FROM students

In the above statement, the wildcard character * is used to query the data of all columns in the students table.

If you only need to query specific columns in the students table, you can use the following statement:

SELECT name,age FROM students

In the above statement, only the name and age columns in the students table are queried.

2. WHERE statement

The WHERE statement is used to conditionally restrict the query results of the SELECT statement. The syntax is:

SELECT column_name(s) FROM table_name WHERE condition

Among them, condition is a conditional expression, and multiple conditions can be combined using logical operators (AND, OR, NOT).

For example, to query the names and ages of students who are 18 years or older in the students table, you can use the following statement:

SELECT name,age FROM students WHERE age>=18

In the above statement, the conditional expression age>=18 is used, and the query Data in the students table that meets this condition.

3. ORDER BY statement

The ORDER BY statement is used to sort the query results of the SELECT statement. The syntax is:

SELECT column_name(s) FROM table_name ORDER BY column_name(s) ASC|DESC

Among them, column_name is the column name that needs to be sorted, which can be one or more column names. Multiple column names are separated by commas; ASC means ascending order, and DESC means descending order.

For example, to query all the data in the students table and sort it in ascending order by age, you can use the following statement:

SELECT * FROM students ORDER BY age ASC

In the above statement, ascending order is used, that is, the data in the AGE column Arrange from smallest to largest.

4. LIMIT statement

The LIMIT statement is used to limit the number of query results of the SELECT statement. The syntax is:

SELECT column_name(s) FROM table_name LIMIT number

where number is the number of records to be queried.

For example, to query the names and ages of the top five students under 18 years old in the students table, you can use the following statement:

SELECT name,age FROM students WHERE age<18 LIMIT 5

In the above statement, the WHERE statement is used to query the results. Limit, and use the LIMIT statement to control the number of query results.

To sum up, SELECT, WHERE, ORDER BY and LIMIT statements are commonly used query statements in PHP. We can use them to accurately query and sort the data in the database and present it to users in a specific format to meet the needs of various web applications.

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