update a table based on the outcome of a join in MySQL - mysql

Question

I understand the syntax here:
UPDATE table
SET column1 = expression1,
column2 = expression2,
...
WHERE conditions;
and here:
UPDATE table1
SET column1 = (SELECT expression1
FROM table2
WHERE conditions)
WHERE conditions;
...but what if I want to update
UPDATE table1
SET column1 = expression
if we get a particular result on a join between table1 and table2
which has WHERE conditions?
I can't seem to figure it out, and I get syntax errors in all of my attempts. Any advice greatly appreciated.

Answer 1

The syntax in MySQL is:
UPDATE table1 JOIN
table2
ON conditions
SET table1.column1 = table2.expression1
WHERE conditions;

Answer 2

What I do is write it as a SELECT statement first.
SELECT t.id
, t.col AS old_val
, s.expr AS new_val
FROM target_table t
JOIN source_table s
ON s.somecol = t.somecol
AND s.othercol < 1
WHERE s.something_else = 'abc'
In this example, t.col is the column that I (eventually) want to assign a new value to. (Used in the SELECT statement here, this just displays the value currently stored in the column.)
The expression s.expr represents the expression that returns the value I want to assign to col. This could be as simple as a column reference, or can be a more complex expression.
The rest of the statement are the normal FROM, JOIN, ON and WHERE clauses we're familiar with.
Once I have a SELECT statement that is working, I can convert that into an UPDATE statement by
replacing SELECT ... FROM (at the beginning of the statement) with the keyword UPDATE
adding a SET clause before the WHERE clause.
For example, I would convert the SELECT statement above to something like this, to assign the value of s.expr to t.col:
UPDATE target_table t
JOIN source_table s
ON s.somecol = t.somecol
AND s.othercol < 1
SET t.col = s.expr
WHERE s.something_else = 'abc'
This the approach that works for me. Writing it as a SELECT first allows me to test, muck with the conditions and expressions, and review to verify which rows are going to be updated, what values are being replaced, and what values are going to be assigned to each row.