Question

This statement

SELECT * FROM TempTable t1 

gives me this output:

id  idIndice    valor
----------------------------
13  11          11111111
13  12          11111
14  11          11111111
14  12          11111
16  12          11111

Now with a little change:

SELECT * FROM TempTable t1
WHERE (select count(*) from TempTable t2 where t1.id = t2.id AND t1.valor != t2.valor) = 1

I get this

id  idIndice    valor
----------------------------
13  11          11111111
13  12          11111
14  11          11111111
14  12          11111

t1 and t2 are references to the same tempTable and I can understand t1.id = t2.id but this:

t1.valor != t2.valor 

is just too much for me. How come same columns as t1 and t2 are references for the same tables can be different?

And why this “= 1” at the end

Answer 1

What the second query essentially does is “Select any rows for which the number of rows with the same ID and a different valor is 1” i.e. “Select any rows that have duplicate id’s“

That is why both rows with id=13 are selected, as they are each others duplicate, and also why both rows for id=14 are selected.

Note that if there are three rows with a same ID, none of them will be selected, because their number of duplicate id’s wont be 1, but 2.