SQL: how to limit a join on the first found row?

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How to make a join between two tables but limiting to the first row that meets the join condition ?

In this simple example, I would like to get for every row in table_A the first row from table_B that satisfies the condition :

select table_A.id, table_A.name, table_B.city 
from table_A join table_B 
on table_A.id = table_B.id2
where ..

table_A (id, name)
1, John
2, Marc

table_B (id2, city)
1, New York
1, Toronto
2, Boston

The output would be:
1, John, New York
2, Marc, Boston

May be Oracle provides such a function (performance is a concern).

The key word here is FIRST. You can use analytic function FIRST_VALUE or aggregate construct FIRST. For FIRST or LAST the performance is never worse and frequently better than the equivalent FIRST_VALUE or LAST_VALUE construct because we don't have a superfluous window sort and as a consequence a lower execution cost:

select table_A.id, table_A.name, firstFromB.city 
from table_A 
join (
    select table_B.id2, max(table_B.city) keep (dense_rank first order by table_B.city) city
    from table_b
    group by table_B.id2
    ) firstFromB on firstFromB.id2 = table_A.id 
where 1=1 /* some conditions here */
;

Since 12c introduced operator LATERAL, as well as CROSS/OUTER APPLY joins, make it possible to use a correlated subquery on right side of JOIN clause:

select table_A.id, table_A.name, firstFromB.city 
from table_A 
cross apply (
    select max(table_B.city) keep (dense_rank first order by table_B.city) city
    from table_b
    where table_B.id2 = table_A.id 
    ) firstFromB
where 1=1 /* some conditions here */
;

4 Ways to Join Only The First Row in SQL, 4 Ways to Join Only The First Row in SQL. Use Correlated Subqueries when the foreign key is indexed. Use a Complete Subquery when you don't have indexes. Use Nested Subqueries if you have an ordered ID column. Use Nested Subqueries if you have an ordered ID column. Use Window Functions if you need more control. over (partition by user_id order by created_at desc specifies a sub-table, called a window, per user_id, and sorts those windows by created_at desc. row_number() returns a row’s position within its window. Thus the first widget for each user_id will have row_number 1. In the outer subquery, we select only the rows with a row_number of 1.

If you want just single value a scalar subquery can be used:

SELECT
    id, name, (SELECT city FROM table_B WHERE id2 = table_A.id AND ROWNUM = 1) city
FROM
    table_A

How to select first row from a join that returns multple rows on the , Presently I get multiple rows per person because they have multiple emails. I am running SQL-Server 2005. EDIT: This is T-SQL. First email is literally the first email� SQL HOME SQL Intro SQL Syntax SQL Select SQL Select Distinct SQL Where SQL And, Or, Not SQL Order By SQL Insert Into SQL Null Values SQL Update SQL Delete SQL Select Top SQL Min and Max SQL Count, Avg, Sum SQL Like SQL Wildcards SQL In SQL Between SQL Aliases SQL Joins SQL Inner Join SQL Left Join SQL Right Join SQL Full Join SQL Self Join SQL

select table_A.id, table_A.name,
FIRST_VALUE(table_B.city) IGNORE NULLS 
         OVER (PARTITION BY table_B.id2 ORDER BY table_B.city) AS "city"
from table_A join table_B 
on table_A.id = table_B.id2
where ..

Ask TOM "Join of two tables and want first row of matching ", Join of two tables and want first row of matching records in second table NVL(n .name,'Not Found') FROM g LEFT OUTER JOIN n USING (a) WHERE d� If you are using Oracle 12c, performing top-N and SQL limit queries is a lot easier with the introduction of the Row Limiting clause. (Related: Oracle 12c New Features for Developers ) The syntax for this clause looks a little complicated (as shown in the official documentation ), but it’s simple to use when you know what data you want to see:

Query:

SELECT a.id,
       a.name,
       b.city
FROM   table_A a
       INNER JOIN
       ( SELECT id2,
                city
         FROM   (
           SELECT id2,
                  city,
                  ROW_NUMBER() OVER ( PARTITION BY id2 ORDER BY NULL ) rn
           FROM   Table_B
         )
         WHERE rn = 1
       ) b
       ON ( a.id = b.id2 )
--WHERE  ...

Outputs:

        ID NAME CITY   
---------- ---- --------
         1 John New York 
         2 Marc Boston   

SQL joins and how to use them, Different joins available in SQL are explained -- inner, left, right, and cross joins. LIMIT and OFFSET � DISTINCT � Functions � GROUP BY � Exercises This would return us the first two rows from that table, containing information such as As you write more queries, you may find that there is more than one way to write a� For example, LIMIT 10 would return the first 10 rows matching the SELECT criteria. This is where sort order matters so be sure to use an ORDER BY clause appropriately. OFFSET offset_value Optional. The first row returned by LIMIT will be determined by offset_value.

On Oracle12c there finally is the new cross/outer apply operator that will allow what you asked for without any workaround.

the following is an example that looks on dictionary views for just one of the (probably)many objects owned by those users having their name starting with 'SYS':

select *
from (
        select USERNAME
        from ALL_USERS
        where USERNAME like 'SYS%'
    ) U
    cross apply (
        select OBJECT_NAME
        from ALL_OBJECTS O
        where O.OWNER = U.USERNAME
            and ROWNUM = 1
    )

On Oracle 11g and prior versions you should only use workarounds that generally full scan the second table based on IDs of the second table to get the same results, but for testing puposes you may enable the lateral operator (also available on 12c without need of enabling new stuff) and use this other one

-- Enables some new features
alter session set events '22829 trace name context forever';

select *
from (
        select USERNAME
        from ALL_USERS
        where USERNAME like 'SYS%'
    ) U,
    lateral (
        select OBJECT_NAME
        from ALL_OBJECTS O
        where O.OWNER = U.USERNAME
            and ROWNUM = 1
    );

Top-N queries: fetch only the first N rows, LIMIT, TOP, or FETCH FIRST abort an SQL query after finding a number of rows. Use the right index. If you switch to a right join and order by: .product_title ASC, .photo_order ASC I think you will get one row per product and it will be the first photo. You will not see products that do not have a photo (which may be an issue). It should be a far simpler query though. – TheJacobTaylor Jun 5 '10 at 1:43

SQL SELECT TOP, LIMIT, ROWNUM, SQL TOP, LIMIT and ROWNUM Examples. The following SQL statement selects the first three records from the "Customers" table (for SQL Server/MS Access):� what I need to do is join these two tables on ASSIGNMENT=ASSIGNMENT_GROUP but only have 1 row returned. when I do a left join I get three rows returned beaucse that is the nature of hte left join sql oracle join limit

Oracle INNER JOIN Demonstrated with Practical Examples, This tutorial shows you how to use the Oracle INNER JOIN clause to retrieve PL/SQL; Functions It means that for each row in the orders table, you can find one or more rows First, specify the main table in the FROM clause, T1 in this case. you should limit the number of joined tables to avoid the performance issue. If you skip it, then offset is 0 and row limiting starts with the first row. The offset must be a number or an expression that evaluates to a number. The offset is subjected to the following rules: If the offset is negative, then it is treated as 0. If the offset is NULL or greater than the number of rows returned by the query, then no row is

MySQL LIMIT, The offset specifies the offset of the first row to return. first 5 rows. Similarly, this example uses the LIMIT clause to find 5 customers who have the lowest credits:. Tip: If you would use a LEFT JOIN in the last query, don’t place a “row_num = 1” condition in the WHERE clause, only after JOIN … ON. If you place it in WHERE clause, SQL Server will make an left outer join, and then filter rows (all NULL values will be rejected here). So you will get an equivalent of inner join.

Comments
  • select * from table_A join (select * feom table_B group by id2) b on table_A.id = b.id2 where ..
  • Join on a subqery with row_number and in the join condition add and row_number=1
  • Providing an example of expected output from a given data(dummy table data) will help us in understanding your requirement.
  • What do you mean by "first"? Tables have no order.
  • This can have a significant performance impact, running a subquery for every row returned from table_A.
  • It depends on table_A.id distinct values and their distribution. Search for "scalar subquery caching". In very many cases it's perfectly fine approach.