SQL Inner Join Performance Data Duplication

Question

Table -- > qt

qstnId	tagID
1	1
1	2
2	3
2	4
2	2

Table-->Question

qid
1
2

qid is the primary key, qstnId is Fk

Now when I run the query -->

mysql> select tagId from qt inner join question on qt.qstnId = 1;

It’s back;

tagID
2
1
2
1

My question is why am I getting duplicate data here.

Answer 1

filter

qt.qstnId = 1 is a filter clause; it belongs to the WHERE clause.

relation

qt.qstnId = Question.qid Describes how tables are related through JOIN. It belongs to the ON clause after JOIN.

Fix your query; if you still have concerns; provide SHOW CREATE TABLE so we can see if you have the necessary indexes (for performance).