Select first record in a One-to-Many relation using left join

Question

I\'m trying to join two tables using a left-join. And the result set has to include only the first record from the \"right\" joined table.

Lets say I have two tables

Answer 1

In Oracle you can do:

WITH first_b AS (SELECT code, min(rowid) AS rid FROM b GROUP BY code)) 
SELECT a.code, a.emp_no, b.city, b.county
FROM a 
INNER JOIN first_b 
 ON first_b.code = a.code
INNER JOIN b
 ON b.rowid = first_b.rid

Answer 2

I modified the answer from ruakh and this seem to work perfectly with mysql.

SELECT 
   table_a.code,
   table_a.emp_no,
   table_b.city,
   table_b.county
FROM table_a a
LEFT JOIN table_b b
ON b.code = a.code 
AND b.id = (  SELECT id FROM table_b 
              WHERE table_b.code = table_a.code 
              LIMIT 1
           )
;

Answer 3

If you are on SQL Server 2005 or later version, you could use ranking to achieve what you want. In particular, ROW_NUMBER() seems to suit your needs nicely:

WITH B_ranked AS (
  SELECT
    *,
    rnk = ROW_NUMBER() OVER (PARTITION BY code ORDER BY city)
  FROM B
)
SELECT
  A.code,
  A.emp_no,
  B.city,
  B.county
FROM A
  LEFT JOIN B_ranked AS B ON A.code = B.code AND b.rnk = 1

OR

WITH B_unique_code AS (
  select * from(
     SELECT
      *,
      rnk = ROW_NUMBER() OVER (PARTITION BY code ORDER BY city)
      FROM B
     ) AS s
  where rnk = 1
)
SELECT
  A.code,
  A.emp_no,
  B.city,
  B.county
FROM A
  LEFT JOIN B_unique_code AS B ON A.code = B.code

Answer 4

After playing around a bit, this turns out to be trickier than I'd expected! Assuming that table_b has some single column that is unique (say, a single-field primary key), it looks like you can do this:

SELECT table_a.code,
       table_a.emp_no,
       table_b.city,
       table_b.county
  FROM table_a
  LEFT
  JOIN table_b
    ON table_b.code = table_a.code
   AND table_b.field_that_is_unique =
        ( SELECT TOP 1
                 field_that_is_unique
            FROM table_b
           WHERE table_b.code = table_a.code
       )
;

Answer 5

Another option: OUTER APPLY

If supported by the database, OUTER APPLY is an efficient and terse option.

SELECT *
FROM 
    Table_A a
OUTER APPLY
    (SELECT TOP 1 * 
    FROM Table_B b_1
    WHERE b_1.code = a.code
    ) b
;

This results in a left join to the indeterminate first matched record. My tests show it to be quicker than any other posted solution (on MS SQL Server 2012).

Answer 6

this is how:

 Select * From TableA a
     Left Join TableB b
         On b.Code = a.Code 
             And [Here put criteria predicate that 'defines' what the first record is]

Hey, if the city and county are unique, then use them

   Select * From TableA a
     Left Join TableB b
         On b.Code = a.Code 
             And b.City + b.county =
                  (Select Min(city + county)
                   From TableB 
                   Where Code = b.Code)

But the point is you have to put some expression in there to tell the query processor what it means to be first.