用php连接postgres上的表

I am still new to postgresql database , do not understand is compared with mysql . how combine 3 table in postgresql,

table1 = field_data_field_smcc_id

table2 = field_data_field_source_isolation

table3 = field_data_field_organism

<?php

$connectString = 'host=' . $host . ' dbname=' . $database . 
    ' user=' . $user . ' password=' . $password;


$link = pg_connect ($connectString);
if (!$link)
{
    die('Error: Could not connect: ' . pg_last_error());
}

$query = 'select * from field_data_field_smcc_id';
$result = pg_query($query);


?>

see this link.i hope meet your answer.