Use Query Result Row in Another Query

Question

I'm trying to use the row from a query in another query.

This query correctly displays the username of the user currently signed in:

$param = $fgmembersite->UserEmail();

$query = mysqli_query($con, "SELECT username FROM Users1 WHERE email = '$param'
");

while ($row = mysqli_fetch_array($query)){
echo $row['username'] ;
    }

I'm trying to find a way to use $row['username'] in another query something like...

$sql = mysqli_query($con, "SELECT * FROM messages WHERE to_user = '" . $row['username'] . "' ");

This doesn't give me a coding error, but it doesn't work. The username row obviously can't be taken from a separate query the way I'm attempting.

I have tried every combination I can think of but nothing has worked for me. Any help greatly appreciated.

Answer 1

尝试子查询

SELECT * FROM messages WHERE to_user in (SELECT username FROM Users1 WHERE email = '$param')

Answer 2

Well you should place your second query inside while:

while ($row = mysqli_fetch_array($query)){
  echo $row['username'] ;
  $sql = mysqli_query($con, "SELECT * FROM messages WHERE to_user = 
    '" . $row['username'] . "' ");
}

Now loop ends when mysqli_fetch_arrray returns NULL and that NULL you are trying to insert into second query.

Answer 3

您可以将两个查询合并为一个。

SELECT * FROM messages WHERE to_user = (SELECT username FROM Users1 WHERE email = '$param')

Answer 4

You can join the queries into one:

SELECT `messages`.*
FROM `messages`
    JOIN `Users1`
        ON `Users1`.`username`=`messages`.`to_user`
WHERE
    `Users1`.`email`='$param'