How to query two tables in the same database in php

In database design, it is often necessary to perform data queries between different tables to obtain the required data information. As a widely used programming language, PHP provides many query methods to achieve this goal. In this article, we will introduce how to query two tables in the same database, aiming to help beginners better master this skill.

Step one: Understand SQL join

To query data in two different tables, we need to use SQL join query. SQL join queries essentially join two or more tables to form a larger result set. In the SQL language, there are three types of join queries: inner joins, left joins and right joins. These types differ in how they join tables and the range of results they return.

Inner join is the most commonly used join method. It only returns results with matching rows between two tables. Left joins and right joins are similar to inner joins, except that they return a wider range of results. A left join includes all rows from the table on the left, while a right join returns all rows from the table on the right.

Step 2: Write the join query statement

We can use the SELECT statement to implement the join query. The key to the query is how to join the two tables together. This depends on the relationship and requirements between the data. Generally, we need to know whether the two tables have the same columns. If so, you can use the equal sign in the WHERE clause to match the two tables. If not, we need to use JOIN and ON statements.

The Join clause defines the table to be used, and the ON clause defines the conditions for joining the tables. In the PHP language, there are two commands that can implement join queries: mysqli_query and PDO::query. No matter which command is used, we must first connect the data.

The following is a basic SQL join query sample:

SELECT * FROM table1

JOIN table2

ON table1.col = table2.col

This query returns matching rows between Table 1 and Table 2. This is a simple inline join query example. In this example, we only query the number of matching rows between the two tables through a passed parameter.

Step 3: Use PHP to implement join query

It is very simple to implement join query in PHP. We only need to follow the following steps:

1. Connect to the database and select the data table to operate

2. Write a SQL join query statement

3. Use mysqli_query or PDO ::query performs query operations

4. Use while loop and mysqli_fetch_array or PDO::fetchAll to obtain results

The following is a simple example of using mysqli_query and while loop to implement SQL join query:

$connect=mysqli_connect("localhost","username","password","dbname");

if(!$connect){

die("connection failed:".mysqli_connect_error());

}

$sql="SELECT *

FROM table1

JOIN table2

ON table1.col=table2.col ";

$result=mysqli_query($connect,$sql);

if(mysqli_num_rows($result)>0){

    while($row=mysqli_fetch_array($result)){

        echo $row['column_name'];

    }

}else{

    echo "No matching rows found.";

}

?>

Summary

The above is an introduction to how to use PHP to query two tables in the same database. SQL join query is the basic method of querying data between data tables. By using the appropriate join type, it becomes easier to join data between multiple data tables and get the information you need. At the same time, it is also very easy to implement join queries using PHP language. I hope this article can help beginners learn and master this skill better.

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