How to update Table 1 from multiple other tables when they have nothing in common?

Question

I have a main table and 3 other tables. Each of these tables has a single row only. I would like to update columns of TableMain with corresponding columns from other tables. And these tables don't have any columns that I can join.

create table TableMain(a1 int, a2 int, b1 int, b2 int, c1 int, c2 int)
create table TableA(a1 int, a2 int)
create table TableB(b1 int, b2 int)
create table TableC(c1 int, c2 int)

I have the following, which works, but looks horrible:

update TableMain
set a1 = (select a1 from TableA),
    a2 = (select a2 from TableA),
    b1 = (select b1 from TableB),
    b2 = (select b2 from TableB),
    c1 = (select c1 from TableC),
    c2 = (select c2 from TableC)

I've tried the code below but I am getting NULLs, so this approach doesn't work:

update TableMain
set a1 = ta.a1, a2 = ta.a2, 
    b1 = tb.b1, b2 = tb.b2, 
    c1 = tc.c1, c2 = tc.c2, 
from TableA ta, TableB tb, TableC tc

Is there a more elegant way to update these columns?

P.S. I asked a similar question, but it addressed updating from a single table.

Answer 1

Consider:

UPDATE tm
SET tm.a1 = ta.a1,
    tm.a2 = ta.a2, 
    tm.b1 = tb.b1, 
    tm.b2 = tb.b2, 
    tm.c1 = tc.c1, 
    tm.c2 = tc.c2
FROM TableMain tm
CROSS JOIN TableA ta
CROSS JOIN TableB tb
CROSS JOIN TableC tc