在一个查询中从多个表中选择记录

Question

I have 4 tables not related to each other. I would like to get few records from these tables using one query instead of 4 of them.

At the moment this is being done with 4 queries. Code:

<?php

$result1 = $mysqli->query("SELECT val1, val2 FROM `table1` WHERE id='1'") or die(mysqli_error());
    $row1 = mysqli_fetch_array($result1, MYSQLI_ASSOC);
    $val1 = $row1['val1'];
    $val2 = $row1['val2'];

$result2 = $mysqli->query("SELECT val3, val4 FROM `table2` WHERE id='2'") or die(mysqli_error());
    $row2 = mysqli_fetch_array($result2, MYSQLI_ASSOC);
    $val3 = $row2['val3'];
    $val4 = $row2['val4'];  

$result3 = $mysqli->query("SELECT val5, val6 FROM `table3` WHERE id='3'") or die(mysqli_error());
    $row3 = mysqli_fetch_array($result3, MYSQLI_ASSOC);
    $val5 = $row3['val5'];
    $val6 = $row3['val6'];  

$result4 = $mysqli->query("SELECT val7, val8 FROM `table4` WHERE id='4'") or die(mysqli_error());
    $row4 = mysqli_fetch_array($result4, MYSQLI_ASSOC);
    $val7 = $row4['val7'];
    $val8 = $row4['val8'];

?>

and my unsuccessful approach to make it in one query. Code:

<?php
$resultglobal = $mysqli->query("
SELECT
tab1.val1 AS val1,
tab1.val2 AS val2,
tab2.val3 AS Val3,
tab2.val4 AS val4,
tab3.val5 AS val5,
tab3.val6 AS val6,
tab4.val7 AS val7,
tab4.val8 AS val8

FROM 

table1 as tab1, 
table2 as tab2,
table3 as tab3,    
table4 as tab4

WHERE 

tab1.id='1', 
tab2.id='2', 
tab3.id='3', 
tab4.id='4' 
") or die(mysqli_error());

$rowg = mysqli_fetch_array($resultglobal, MYSQLI_ASSOC);
$val1 = $rowg['val1'];
$val2 = $rowg['val2']; 
$val3 = $rowg['val3'];
$val4 = $rowg['val4'];
$val5 = $rowg['val5'];
$val6 = $rowg['val6']; 
$val7 = $rowg['val7'];
$val8 = $rowg['val8'];   
?>

What's wrong? What will be a solution here? Any advice highly appreciated. Thank you very much in advance.

Answer 1

You just need to replace , WITH AND in your WHERE clause. Like this:

WHERE
tab1.id='1' AND 
tab2.id='2' AND
... 

It will work ok as long as ID is unique (eg primary key). It is an implicit CROSS JOIN.

Answer 2

You could do a multi-query this way:

$statement = "SELECT val1, val2 FROM `table1` WHERE id='1';";
$statement .= "SELECT val3, val4 FROM `table2` WHERE id='2';";
$statement .= "SELECT val5, val6 FROM `table3` WHERE id='3';";
$statement .= "SELECT val7, val8 FROM `table4` WHERE id='4'";

$values = $mysqli -> multi_query($statement) or die ('Error: '.$mysqli -> error);

while ($row = $values -> fetch_assoc()) {

    $val1 = $row['val1'];
    $val2 = $row['val2'];
    $val3 = $row['val3'];
    $val4 = $row['val4'];
    $val5 = $row['val5'];
    $val6 = $row['val6'];
    $val7 = $row['val7'];
    $val8 = $row['val8'];

}