How to show the repeated value as NULL in sql? - mysql

Question

I have a query which gives result as below, how to replace duplicate values with NULL
Query:
SELECT
word.lemma,
synset.definition,
synset.pos,
sampletable.sample
FROM
word
LEFT JOIN
sense ON word.wordid = sense.wordid
LEFT JOIN
synset ON sense.synsetid = synset.synsetid
LEFT JOIN
sampletable ON synset.synsetid = sampletable.synsetid
WHERE
word.lemma = 'good'
Result:
Required Result: all the greyed out results as NULL

Answer 1

First, this is the type of transformation that is generally better done at the application level. The reason is that it presupposes that the result set is in a particular order -- and you seem to be assuming this even with no order by clause.
Second, it is often simpler in the application.
However, in MySQL 8+, it is not that hard. You can do:
SELECT w.lemma,
(CASE WHEN ROW_NUMBER() OVER (PARTITION BY w.lemma, ss.definition ORDER BY st.sample) = 1
THEN ss.definition
END) as definition,
ss.pos,
st.sample
FROM word w LEFT JOIN
sense s
ON w.wordid = s.wordid LEFT JOIN
synset ss
ON s.synsetid = ss.synsetid LEFT JOIN
sampletable st
ON ss.synsetid = st.synsetid
WHERE w.lemma = 'good'
ORDER BY w.lemma, ss.definition, st.sample;
For this to work reliably, the outer ORDER BY clause needs to be compatible with the ORDER BY for the window function.

Answer 2

If you are using Mysql 8 try with Rank().. As I didn't have your table or data couldn't test this query.
SELECT
word.lemma
,case when r = 1 synset.definition else null end as definition
,synset.pos
,sampletable.sample
FROM
(
SELECT
word.lemma
,synset.definition
,synset.pos
,sampletable.sample
,RANK() OVER (PARTITION BY synset.definition ORDER BY synset.definition) r
FROM
(
SELECT
word.lemma,
synset.definition,
synset.pos,
sampletable.sample
FROM
word
LEFT JOIN
sense ON word.wordid = sense.wordid
LEFT JOIN
synset ON sense.synsetid = synset.synsetid
LEFT JOIN
sampletable ON synset.synsetid = sampletable.synsetid
WHERE
word.lemma = 'good'
) t
)t1;