how to do a SELECT with a composite key?

问题

I have a table with a composite key, and I want to do my SELECT query with that key, is there a way to put a name to the composite key or to do the query without having to add an AND

let's say:

SELECT * FROM MyTable WHERE PRIMARY KEY = 12345

being that primary key a composite key

回答1:

No, there is no concept of SELECT * FROM MyTable WHERE PRIMARY KEY = 'PrimaryKeyName'. In other words, the predicate (WHERE clause) does not allow for this. You will need to list all columns that are contained within the composite primary key in the predicate.

A potential solution is to build a table that stores all primary key combinations for the fact table. You can add an identity field to that table and make it a foreign key constraint on your table. Then, your predicate uses the single foreign key column.

回答2:

There are other ways you could write equivalent logic. For instance:

where not (key1 <> val1 or key2 <> val2)

or:

select t.*
from (select t.* from t where key1 = val1) t
where t.key2 = val2

or perhaps:

where t.key1 + '|' + t.key2 = val1 + '|' + val2

(assuming | is not ever in either value).

or:

where (case when t.key1 <> val1 then 0
            when t.key2 <> val2 then 0
            else 1
       end) = 1

or even:

where (case when t.key1 = val1 then 1 end) = (case when t.key2 = val2 then 1 end)

But there is no reasonable alternative to using AND.

I should note that tables that I create almost always have identity columns as primary keys. I'm not a fan of composite primary keys, so you might want to use an identity column.

回答3:

Maybe Computed column is the thing you are looking for.

Briefly, you can have another column, which is system generated based on your definition (here is the combination of each column of your composite key).

Then, when you query the table, you can query that Computed Column.

For example, for a Charge: you have ChargeCode and ChargeAmount, and those two are the composite key. Then the Computed Column will have the value (ChargeCode + ChargeAmount).

Charge example one: ChargeCode: A ChargeAmount: 100 (Composite key) If you write a query as usual: SELECT * FROM Table WHERE ChargeCode = 'A' AND ChargeAmount = '100'

If with Computed Column: ComputedColumn_Key: A100 (notice the data type conversion here for string concatenation)

Then the query will be SELECT * FROM TABLE WHERE ComputedColumn_Key = 'A100'

回答4:

thanks to everybody for the brainstorm, it helped me.

I ended making a view concatenating the columns of the composite key in one new column and doing the SELECT query to that view.

view:

SELECT (CONVERT(VARCHAR,PK1) + CONVERT(VARCHAR, PK2)) AS NEW_ID, nameColumn FROM MyTable

query

SELECT nameColumn FROM viewName WHERE NEW_ID = 12345

来源：https://stackoverflow.com/questions/49654259/how-to-do-a-select-with-a-composite-key