使用哪种类型的JOIN

Question

What type of JOIN would I use to get table1 and table2 to be matched only once. For example, I have table1 (40 rows) and table2 (10000 rows). But I get table1 repeated over and over when I use a join on table1.LocationArea = table2.Location

What I get:                         What I wish I could get:
t1.LocationArea,t2.Location         t1.LocationArea,t2.Location
---------------------------         ---------------------------
az,az                               az,az
az,az                               null,az
ca,ca                               ca,ca
il,il                               il,il
tx,tx                               tx,tx
tx,tx                               null,tx
az,az                               null,az
                                    null,il
                                    null,ca

I wish to end up with 10000 records in the query.

I have tried inner join , left , and I am using ZOHO reports which does not support outer join's.

SELECT "table1"."LocationArea", "Location" 
FROM "table2"
left join "table1" on  "Location" = "table1"."LocationArea"

Answer 1

Obviously, you have duplicate values for both of the joining columns. Instead of the Cartesian product an [INNER] JOIN would produce for this, you want each row to be used only once . You can achieve this by adding a row number ( rn ) per duplicate and join on rn additionally.

Each table can have more or fewer dupes for the same value than the other unless you have additional restrictions in place (like a FK constraint) - but there is nothing in your question. To keep all rows one would use a FULL [OUTER] JOIN . But you want to keep 10000 records in the result, which is the cardinality of table2 . So it must be a LEFT [OUTER] JOIN on table1 (with 40 rows) - and exclude possible excessive rows from table1 .

SELECT t1."LocationArea", t2."Location"
FROM  (
   SELECT "Location"
        , row_number() OVER (PARTITION BY "Location") AS rn
   FROM   table2
   ) t2
LEFT JOIN (
   SELECT "LocationArea"
        , row_number() OVER (PARTITION BY "LocationArea") AS rn
   FROM   table1
   ) t1 ON t1."LocationArea" = t2."Location"
       AND t1.rn = t2.rn;

Works for Postgres or SQL Server. MySQL doesn't support window functions, you would need a substitute:

• SQL SELECT last entry without limiting

To be clear: LEFT JOIN is just shorthand for LEFT OUTER JOIN , so you are already using an outer join. Your statement is a misunderstanding :

I am using ZOHO reports which does not support outer join's.

Answer 2

SELECT * FROM table1 LEFT JOIN table2 ON `table_1_primary_key` = `table_2_primary_key`

Or SELECT colname FROM table1 LEFT JOIN table2 ON table_1.colname = table_2.colname

Depending on the structure of your database