如果一列为空，则使用默认值在两列上进行外部联接

Question

I have two tables - one a transaction table, and the other a look-up table. These tables join on two columns, one of which always has a value, while the other may not.

Sample this

transaction table

category | subcategory | marks A | 01 | 10 A | 02 | 20 B | 03 | 30 B | 04 | 40 C | 05 | 50

lookup table

category | subcategory | cut-off A | | 15 A | 01 | 25 B | 03 | 35 B | | 55 C | | 75

I wish to get the cut-off next to each entry in the transaction table by joining the category and subcategory columns. For cases where subcategory does not have an exact match, the record corresponding to null needs to be picked.

Desired output format:

output format

category | subcategory | marks | cut-off A | 01 | 10 | 25 A | 02 | 20 | 15 B | 03 | 30 | 35 B | 04 | 40 | 55 C | 05 | 50 | 75

I have been trying to think along the lines of a query like the one below, the problem of course is it doesn't work the way I want it to as it doesn't handle the null case right.

select t.category, t.subcategory, t.marks, l.cut-off from transaction t left outer join lookup l on t.category = l.category and t.subcategory = l.subcategory

Do I need multiple queries (like a union all that handles null and not null separately? Are there options that work with single query?

Answer 1

您可以这样做：

SELECT t.category, t.subcategory, t.marks, l.cut-off FROM transaction t left outer join lookup l on t.category = l.category and t.subcategory = ISNULL(l.subcategory, t.subcategory)

Answer 2

Using a sub query (NVL for oracle)

SELECT t.category,
  t.subcategory,
  t.marks,
  NVL( 
      (SELECT l.cut_OFF from lookup l where l.category = t.category and l.subcategory = t.subcategory), 
      (SELECT l.cut_OFF from lookup l where l.category = t.category and l.subcategory is null))
FROM transaction t