update from another table but join on calculated column

Question

Table A has three columns; an IP address(varchar), lab ID(int), and origin (int).

Table B has two columns; an ID(int) and labName(varchar)

The labName is table B is made up of the IP address' first octet in A. For instance if the IP was 192.168.5.18 then there would be a lab name 'lab 192'. What I need to do is update Table A's labID column with the matching ID in table B.

I already have parsing done to build the lab name from the IP:

What I'm not sure how to do is the update based on that calculated field. Code below won't work but I'm hoping it might explain what I'm attempting to do:

UPDATE tableA
SET labID = (
    SELECT labName
    FROM tableB
    WHERE labName = concat(‘lab ‘, substring_index(ip, “.”, 1)
)
WHERE origin = 3

Answer 1

MySQL will permit you to call a function (or any expression) in a join's ON clause using a join for the table_reference in UPDATE syntax

UPDATE
  tableA
  -- Join on the IP substring expression
  INNER JOIN tableB ON tableB.labName = CONCAT('lab ', substring_index(tableA.ip, '.', 1))
-- Set labID to tableB ID
SET tableA.labID = tableB.ID
WHERE origin = 3