Oracle SQL UPDATE query with data and field matching from another table

Question

I have two tables in an Oracle SQL-database. The fields which are of interest are these:

Table A: Reference-A | Type-A | Timestamp-A

Table B: Reference-B | Type-B | Timestamp-B

The timestamp field in Table B is newly created and therefore all NULL, and I'd like to update that field with timestamp values from table A.

Reference-A and reference-B do match one-to-one, but there are multiple rows for each reference with different Type, so I also need to select on the type. Type-A and Type-B aren't the same, so I need to do an IF-ELSE-matching on them, eg: Type-B1 -> Type-A1, Type-B2 -> Type-A2

Not all combinations of Reference-B and Type-B have a match in table A, so for those cases it can just set the timestamp in Table-B to null.

So how does the SQL query for this UPDATE statement look like? I've tried this one (pseudo);

UPDATE B
SET timestamp = (
    SELECT A.timestamp 
    FROM A, B
    WHERE A.reference = B.reference
      AND A.type = 
        (CASE 
          WHEN B.type = 'B1' 
            THEN 'A1'
          WHEN B.type = 'B2' 
            THEN 'A2' 
          WHEN B.type = 'B3' 
            THEN 'A3'
          WHEN B.type = 'B4' 
            THEN 'A4' 
        END)
);

But when I run this query, I get the following error: "SQL Error: ORA-01427: single-row subquery returns more than one row".

Any ideas?

Answer 1

UPDATE B
SET timestamp = (
    SELECT A.timestamp 
    FROM A
    WHERE A.reference = B.reference
      AND A.type = 
        (CASE 
          WHEN B.type = 'B1' 
            THEN 'A1'
          WHEN B.type = 'B2' 
            THEN 'A2' 
          WHEN B.type = 'B3' 
            THEN 'A3'
          WHEN B.type = 'B4' 
            THEN 'A4' 
        END)
      AND rownum < 2 -- !!! select only 1 row
);

Answer 2

As indicated in error message, your sub query is returning more than one row. In the sub query, you don't need to join table B again. If table Reference-A and Reference-B do match one-to-one, you could update your query like this

UPDATE B
SET timestamp = (
    SELECT A.timestamp 
    FROM A
    WHERE A.reference = B.reference
      AND A.type = 
        (CASE 
          WHEN B.type = 'B1' 
            THEN 'A1'
          WHEN B.type = 'B2' 
            THEN 'A2' 
          WHEN B.type = 'B3' 
            THEN 'A3'
          WHEN B.type = 'B4' 
            THEN 'A4' 
        END)
);