How to implement multi-table query in php

How to implement multi-table query in php

Multi-table joint query means that the result of the query needs to be multiple table contents, and establish their relationship as a temporary table.

Multi-table joint query cannot be indexed to optimize the query speed, so it is generally not recommended to use it.

1. Use mysqli_connect to connect to the database

2. Execute multi-table query statements

// 设置编码，防止中文乱码
mysqli_query($conn , "set names utf8");
 
 // 多表查询
$sql = 'select * from table1,table2';
 
mysqli_select_db( $conn, 'DEMO' );
$retval = mysqli_query( $conn, $sql );
if(! $retval )
{
    die('无法读取数据: ' . mysqli_error($conn));
}
while($row = mysqli_fetch_array($retval, MYSQLI_ASSOC))
{
    echo $row;
}
mysqli_close($conn);

More multi-table query methods:

1. Ordinary method

select * from table1,table2

2. left join right join Waiting method

select * from table1 t1 left join table2 t2 on t1.id = t2.id

3, UNION method

select * from table1 union select * from table2

4, Nested query method

select * from table1 where id in (select pid from table2 where pid > 10)

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