Question

The following is a hypothetical situation this which is close to my real problem. Table1

recid   firstname    lastname   company
1       A             B          AAA
2       D             E          DEF
3       G             H          IJK
4       A             B          ABC

I have a table2 that looks like this

recid   firstname    lastname   company
10      A             B          ABC
20      D             E          DEF
30      M             D          DIM
40      A             B          CCC

Now if I join the table on recid, it will give 0 results, there will be no duplicates because recid is unique. But if I join on firstname and lastname columns, which are not unique and there are duplicates, I get duplicates on inner join. The more columns I add on join, the worse it becomes (more duplicates are created).

In the above simple situation, how can I remove duplicates in the following query? I want to compare firstname and lastname, if they match, I return firstname, lastname and recid from table2

SELECT DISTINCT * FROM
(SELECT recid, first, last FROM table1) a
INNER JOIN
(SELECT recid, first,last FROM table2) b
ON a.first = b.first

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The script is here if anyone wants to play with it in future

CREATE TABLE table1 (recid INT NOT NULL PRIMARY KEY, first varchar(20), last varchar(20), company varchar(20))
CREATE TABLE table2 (recid INT NOT NULL PRIMARY KEY, first varchar(20), last varchar(20), company varchar(20))

INSERT INTO TABLE1 VALUES(1,'A','B','ABC')
INSERT INTO TABLE1 VALUES(2,'D','E','DEF')
INSERT INTO TABLE1 VALUES(3,'M','N','MNO')
INSERT INTO TABLE1 VALUES(4,'A','B','ABC')

INSERT INTO TABLE2 VALUES(10,'A','B','ABC')
INSERT INTO TABLE2 VALUES(20,'D','E','DEF')
INSERT INTO TABLE2 VALUES(30,'Q','R','QRS')
INSERT INTO TABLE2 VALUES(40,'A','B','ABC')

Answer 1

You don’t want to do a join per se, you’re merely testing for existence/set inclusion.

I don’t know what current flavor of SQL you’re coding in, but this should work.

SELECT MAX(recid), firstname, lastname 
FROM table2 T2
WHERE EXISTS (SELECT * FROM table1 WHERE firstname = T2.firstame AND lastname = T2.lastname)
GROUP BY lastname, firstname

If you want to implement as a join, leaving the code largely the same:

i.e.

SELECT max(t2.recid), t2.firstame, t2.lastname 
FROM Table2 T2 
INNER JOIN Table1 T1 
    ON T2.firstname = t1.firstname and t2.lastname = t1.lastname
GROUP BY t2.firstname, t2.lastname 

Depending on the DBMS, an inner join may be implemented differently to an Exists (semi-join vs join) but the optimizer can sometimes figure it out anyway and chose the correct operator regardless of which way you write it.