General LINQ queries

Question

say I have 2 columns, ColumnA and ColumnB from a table named TableAB,

with sets of data like this

ColumnA               ColumnB
======               ======
12345                   67890
54321                   67809
......                     ........
67890                   abcde
67809                   fghkm
.......                     .....

I would like to do a LINQ queries to give me results such that when the ColumnB values appear in ColumnA, then ColumnB gives the new values such that

Results
======
12345abcde
54321fghkm

Is this possible?

Appreciate if there is some code queries.

thanks.