php sql UPDATE with nested FROM (SELECT) - php

Question

After hours of trying I need your advice.
I want to combine rows from 2 tables.
After I created a new row in table1 I want to find a row in table2 and combine some of the fields.
If I put the nested SELECT in the SET function
(SET postcode=(SELECT etc)
is works, but if I put it in the FROM function is gives an Error that the syntax is wrong
my code:
$sql = "INSERT INTO instanties(institution, category, postcode)
VALUES('$emapData[0]', '$emapData[1]', '$emapData[2]')";
if ($conn->query($sql) === TRUE) {
//get last added id
$last = $conn->insert_id;
//define WHERE function
$where="postcode_id=$postcode_id AND (minnumber <= $number AND maxnumber >= $number)";
//UPDATE last added row in table with info from other table
$sql2 = "UPDATE instanties
SET postcode_id=pc.postcode_id
FROM
(
SELECT postcode_id
FROM postcode
WHERE $where LIMIT 1
) pc
WHERE id=$last";
$result = $conn->query($sql2);
if ($result) {
echo 'update is done<br/><br/>';
}
}
else {
echo "Error: " . $sql2 . "<br>" . $conn->error.'<br/><br/>';
}
}
else {
echo "Error: " . $sql . "<br>" . $conn->error.'<br/><br/>';
}

Answer 1

That's not a valid MySQL syntax. You cannot add a "FROM" clause to an UPDATE statement.
http://dev.mysql.com/doc/refman/5.0/en/update.html
However, what you want to accomplish is still possible this way:
$sql2 = "UPDATE instanties
SET postcode_id=
(
SELECT postcode_id
FROM postcode
WHERE $where LIMIT 1
)
WHERE id=$last";
As long as there is only 1 result from the nested SELECT (and your LIMIT 1 kinda does that).
EDIT:
If you need many fields from the postcode table, you can join on it:
$sql2 = "UPDATE instanties as i
JOIN (
SELECT *
FROM postcode
WHERE $where LIMIT 1
) as pc
SET i.postcode_id=pc.postcode_id
WHERE i.id=$last";
We would usually use an "ON" clause with the join, but since you're only updating 1 row and your nested SELECT will also only return 1 row, it's not necessary.

Answer 2

try this:
$sql2 = "UPDATE instanties
SET postcode_id=(
SELECT postcode_id
FROM postcode
WHERE $where LIMIT 1)
WHERE id=$last";