How can I improve this conditional UPDATE query? - sql

Question

I have a table t with several columns, let's name them a, b and c. I also have a state column which indicates the current state. There is also an id column.
I want to write the following query: update column a always, but b and c only if the application state is still equal to the database state. Here, the state column is used for optimistic locking.
I wrote this query as following:
UPDATE t
SET a = $a$,
b = (CASE WHEN state = $state$ THEN $b$ ELSE b END),
c = (CASE WHEN state = $state$ THEN $c$ ELSE c END)
WHERE id = $id$ AND
(
a != $a$ OR
b != (CASE WHEN state = $state$ THEN $b$ ELSE b END) OR
c != (CASE WHEN state = $state$ THEN $c$ ELSE c END)
)
Here, $id$, $a$, ... are input variables from the application. The second part of the WHERE clause is to avoid updates which do not effectively update anything.
This query works as expected, but is very clumsy. I am repeating the same condition several times. I am looking for a way to rewrite this query in a more elegant fashion. If this was a simple SELECT query, I could do something with a LATERAL JOIN, but I cannot see how to apply this here.
How can I improve this query?

Answer 1

Split the query in two:
UPDATE t
SET a = $a$
WHERE id = $id$
UPDATE t
SET b = $b$,
c = $c$
WHERE id = $id$ AND
state = $state$
If you need atomicity, wrap in a transaction.

Answer 2

This seems a bit cleaner(untested):
WITH src AS (
SELECT $a$ AS a
, (CASE WHEN state = $state$ THEN $b$ ELSE b END) AS b
, (CASE WHEN state = $state$ THEN $c$ ELSE c END) AS c
FROM t
WHERE id = $id$
)
UPDATE t dst
SET a=src.a, b=src.b, c=src.c
FROM src
WHERE dst.id = src.id
AND (src.a, src.b, src.c) IS DISTINCT FROM (dst.a, dst.b, dst.c)
;

Answer 3

EDIT: It Took me a while to realize my fault here: The question obviously targets at a single update, while my answer tried to update many rows. However, if you need to execute this Update for a set of rows you could:
Insert the needed parameters in a temporary table
Join that table within the "t2" subquery
Select it's columns (e.g. tempTable.b As tempB)
Replace the Parameters (e.g. $b$ -> t2.tempB)
.
UPDATE t
SET a=source.a,
b=source.b,
c=source.c
FROM
(
SELECT
id,
a,
(CASE WHEN UpdateCondition THEN $b$ ELSE b END) AS b,
(CASE WHEN UpdateCondition THEN $c$ ELSE c END) AS c
FROM
(
SELECT state = $state$ As UpdateCondition, * FROM t
) As t2
WHERE
id = $id$ AND
(
a != $a$ OR
b != (CASE WHEN UpdateCondition THEN $b$ ELSE b END) OR
c != (CASE WHEN UpdateCondition THEN $c$ ELSE c END)
) AS source
WHERE t.id=source.id;
The Sub query for t2 gives you your state Condition and executes the calculation for it only once per row.
The subquery for "source" gives you the mapped values and filters those without changes.

Answer 4

The only filter you need is on ID = $id
The case statement says don't change it in the update if the state doesn't match, so you don't need to filter it.
EDIT
where Id = $id and a !=$a
Or (state = $state and (b !=b or c!= $c))
If you do any more than that then"always update a" will not necessary be true.
3rd attempt checks for the possibility of a remaining the same, but b or c updating.