How to Handle NULL Values When Updating Multiple Rows in PostgreSQL?

Casting NULL When Updating Multiple Rows

When updating multiple rows in a table using a single query, it's essential to ensure that the values being assigned match the data types of the columns involved. If NULL values are involved, this can lead to errors due to type mismatches.

Issue Overview

Consider the following query:

UPDATE foo SET x=t.x, y=t.y
FROM (
  VALUES (50, 50, 1),
         (100, 120, 2)
)
AS t(x, y, pkid)
WHERE foo.pkid=t.pkid

This query works for non-NULL values, but when NULL values are introduced, an error occurs:

UPDATE foo SET x=t.x, y=t.y
FROM (
  VALUES (null, 20, 1),
         (null, 50, 2)
)
AS t(x, y, pkid)
WHERE foo.pkid=t.pkid

The error is caused by the missing type specification for the NULL values. PostgreSQL attempts to guess their type based on the literal, resulting in a mismatch with the integer column x.

Solutions

To resolve this issue, several solutions can be employed:

0. Select Row with LIMIT 0, Append Rows with UNION ALL VALUES

UPDATE foo f
SET    x = t.x
     , y = t.y
FROM  (
  (SELECT pkid, x, y FROM foo LIMIT 0) -- parenthesis needed with LIMIT
   UNION ALL
   VALUES
      (1, 20, NULL)  -- no type casts here
    , (2, 50, NULL)
   ) t               -- column names and types are already defined
WHERE  f.pkid = t.pkid;

1. Select Row with LIMIT 0, Append Rows with UNION ALL SELECT

UPDATE foo f
SET    x = t.x
     , y = t.y
FROM  (
  (SELECT pkid, x, y FROM foo LIMIT 0) -- parenthesis needed with LIMIT
   UNION ALL SELECT 1, 20, NULL
   UNION ALL SELECT 2, 50, NULL
   ) t               -- column names and types are already defined
WHERE  f.pkid = t.pkid;

2. VALUES Expression with Per-Column Type

UPDATE foo f
SET    x = t.x
     , y = t.y
FROM  (
   VALUES 
     ((SELECT pkid FROM foo LIMIT 0)
    , (SELECT x    FROM foo LIMIT 0)
    , (SELECT y    FROM foo LIMIT 0))  -- get type for each col individually
   , (1, 20, NULL)
   , (2, 50, NULL)
   ) t (pkid, x, y)  -- columns names not defined yet, only types.
WHERE  f.pkid = t.pkid;

3. VALUES Expression with Row Type

UPDATE foo f
SET x = (t.r).x         -- parenthesis needed to make syntax unambiguous
  , y = (t.r).y
FROM (
   VALUES
      ('(1,20,)'::foo)  -- columns need to be in default order of table
     ,('(2,50,)')       -- nothing after the last comma for NULL
   ) t (r)              -- column name for row type
WHERE  f.pkid = (t.r).pkid;

4. VALUES Expression with Decomposed Row Type

UPDATE foo f
SET    x = t.x
     , y = t.y
FROM (
   VALUES
      (('(1,20,)'::foo).*)  -- decomposed row of values
    , (2, 50, NULL)
   ) t(pkid, x, y)  -- arbitrary column names (I made them match)
WHERE  f.pkid = t.pkid;     -- eliminates 1st row with NULL values

5. VALUES Expression with Types Fetched from Row Type

UPDATE foo f
SET   (  x,   y)
    = (t.x, t.y)  -- short notation, see below
FROM (
   VALUES
      ((NULL::foo).pkid, (NULL::foo).x, (NULL::foo).y)  -- subset of columns
    , (1, 20, NULL)
    , (2, 50, NULL)
   ) t(pkid, x, y)       -- arbitrary column names (I made them match)
WHERE  f.pkid = t.pkid;

The choice of solution depends on factors such as performance, convenience, and the number of columns involved. Solutions 4 and 5 are generally recommended for simplicity and flexibility.

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