Will selecting specific columns affect the number of rows in MySQL?

Question

I have a simple query with a few joins, but when I add columns to my select from the same set of joins, simply adding or removing columns from the select without changing any joins, the number of rows changes Variety.

P粉250422045 · Answer

Yes, they can. For example, your query, or "simple query" as you call it, can have different keywords, and adding a column to a similar select will change the total number of rows in the results.

select distinct t1.id, t2.id
from t1
left join t2 on t1.id = t2.id

VS

select distinct t1.id, t2.id, t2.job
from t1
left join t2 on t1.id = t2.id

This is a demo