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What will be the output of the left join? [closed]

I have 2 tables A and B. Table A has 3 rows and one column having values 1, 1, 1. Table B has 2 rows and one column having values 1, 1. Upon applying left join, Table A left join Table B The output I will receive is: 1, 1, 1.

Is it correct?



No, your assumption is not correct. If you join on the columns you mention, you will get 6 rows (3 x 2), because every row matches each other.

Table t1

id_t1 colx
1 1
2 1
3 1

Table t2

id_t2 coly
10 1
20 1


select *
from t1
left outer join t2 on t2.coly = t1.colx
order by t1.id_t1, t2.id_t2;


id_t1 colx id_t2 coly
1 1 10 1
1 1 20 1
2 1 10 1
2 1 20 1
3 1 10 1
3 1 20 1

You seem to misunderstand what a left outer join does. Each row in table t1 has two matches in table t2. You get these with a left outer join, but you also get them with an inner join. The difference between the two join types is when there is no match. Let’s add another row to table t1:

id_t1 colx
4 2

There is no match in t2 for the value 2. An inner join would not show this row in the results. A left outer join in contrast will add the following row to your results:

id_t1 colx id_t2 coly
4 2 null null
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