如何在同一个表的两个不同列中选择匹配的记录？

Question

I have two table like below, and I need below output. I have around millions of records so I cannot hard code value="***" :

Col1      Col2      Col1_dtm       Col2_dtm  
a         a         01/01/1900    03/01/1900
b         c         01/01/1900    04/01/1900
c1        b         01/01/1900    02/01/1900
d1        g         01/01/1900    01/01/1900
e1        f         01/01/1900    06/12/1900
f         d         01/01/1900    05/01/1900
c                   01/01/1900    01/01/1900
d                   01/01/1900    01/01/1900
e                   01/01/1900    01/01/1900
g                   01/01/1900    01/01/1900

I need output as below:

Col1      Col2      Col2_dtm-col1_dtm
a         a         59 (days)
b         b         31 (days)
c         c         90 (days)
d         d         120 (days)
f         f         162 (days)
g         g         0 (days)

Answer 1

Try this:

select t1.col1,
       t2.col2,
       datediff(day, t1.col1_dtm, t2.col2_dtm)
from my_table t1
join my_table t2 on t1.col1 = t2.col2