选择记录，其中所有行在同一行的两列中具有相同的值

Question

Here is my sample table

Col1 Col2
A      A
B      B
A      C
B      D
C      C

I want to be able to select distinct records where all rows have the same value in Col1 and Col2. So my answer should be

Col1 Col2
A       A
B       B
C       C

Answer 1

Simply:

select distinct * from t where col1 = col2;

if both cols have null and you want to get that row too:

select distinct * from t where coalesce(col1, col2) is null or col1 = col2;

Answer 2

The query is already written in your request:

select distinct records where all rows have the same value in Col1 and Col2

SELECT DISTINCT *
FROM tbl
WHERE Col1 = Col2