Query statements in PHP and how to use them

In PHP, query statements are a necessary component, which can help us retrieve data from the database. This article will introduce common query statements used in PHP and how to use them.

1. SELECT statement

The SELECT statement is a commonly used query statement in PHP. Its role is to select data from the database and return the results to the calling program.

The general syntax of the SELECT statement is as follows:

SELECT column_name(s) FROM table_name

In the SELECT statement, column_name(s) represents the name of the column you want to query. You can Use the * sign to query all columns in the table. FROM table_name represents the name of the table from which the data you want to query comes. For example, to query all data in a customer table:

SELECT * FROM customer

2. WHERE statement

The WHERE statement is a very useful tool , you can filter query results based on some conditions.

The general syntax of the WHERE statement is as follows:

SELECT column_name(s) FROM table_name WHERE condition

In the WHERE statement, condition represents the query condition. For example, to query the data named "Zhang San" in the customer table:

SELECT * FROM customer WHERE name='Zhang San'

You can also use comparison operators in the WHERE statement, for example ：

SELECT * FROM customer WHERE age>=18

3. ORDER BY statement

ORDER BY statement is used to query the results according to the specified column name Sort.

The general syntax of the ORDER BY statement is as follows:

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

In the ORDER BY statement, column_name(s) ) represents the column name by which you want to sort. ASC stands for ascending order and DESC stands for descending order. For example:

SELECT * FROM customer ORDER BY age DESC

4. GROUP BY statement

The GROUP BY statement is used to compare results based on one or more columns Make groups.

The general syntax of the GROUP BY statement is as follows:

SELECT column_name(s) FROM table_name GROUP BY column_name(s)

In the GROUP BY statement, column_name(s) represents the desired The column name to group by. For example:

SELECT age, COUNT(*) FROM customer GROUP BY age

The above query statement will return the number of customers with the same age. For example, if there are 3 customers with age 25, return 3.

5. JOIN statement

The JOIN statement is used to combine different tables together based on the columns in the table.

The general syntax of the JOIN statement is as follows:

SELECT column_name(s) FROM table1 JOIN table2 ON table1.column_name=table2.column_name

In the JOIN statement, table1 and table2 represent The name of the table to be combined, column_name represents the name of the column to be combined. For example, to combine tables named customer and order data:

SELECT customer.name, order.order_id FROM customer JOIN order ON customer.customer_id=order.customer_id

In PHP, use Query statements can obtain the required data from the database. Although this article only introduces some commonly used query statements, we can use more query statements and combinations according to specific needs.

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