table -- > qt
| qstnId | tagId |
|---|---|
| 1 | 1 |
| 1 | 2 |
| 2 | 3 |
| 2 | 4 |
| 2 | 2 |
table --> question
| qid |
|---|
| 1 |
| 2 |
qid is primary key and qstnId is Fk
now when i'm running query -->
mysql> select tagId from qt inner join question on qt.qstnId = 1;
it's returning;
| tagId |
|---|
| 2 |
| 1 |
| 2 |
| 1 |
My question is why am i getting duplicate data here.
CodePudding user response：
Filtering
qt.qstnId = 1 is a filtering clause; it belongs in a WHERE clause.
Relation
qt.qstnId = question.qid states how the tables are related via a JOIN. It belongs in an ON clause after the 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).
CodePudding user response：
SELECT tagId
FROM qt
INNER JOIN question on qt.qstnId = question.qid
WHERE qid=1;