Concatenate two sql tables

I have two tables (Table 1 and Table 2 in the following example) with two columns each. What I am trying to do is:

keep all the entries of Table 1
add the entries from Table 2 if the element of col_3 (in Table 2) is also an element of col_1 (in Table 1)
the Result Table will have three columns: the two original ones from Table 1, and col_4 from Table 2

Table 1

col_1	col_2
1	a
1	b
2	c
3	d

Table 2

col_3	col_4
1	w
1	x
2	y
4	z

Result Table

col_1	col_2	col_4
1	a	NULL
1	b	NULL
2	c	NULL
3	d	NULL
NULL	NULL	w
NULL	NULL	x
NULL	NULL	y

In the example, all the elements of Table 1 are in the Result Table (and populate col_1 and col_2), while only the first three elements of Table 2 are in the Result Table (and populate col_4).

This looks very similar to a left join in some sense. But instead of just creating a new column and adding the values to it, how do I create the new rows and fill them with the entries from Table 2? Maybe some combination of union and join might work.

Solution

You can solve this with a UNION:

SELECT col_1, col_2, NULL as col_3 FROM table1
UNION
SELECT NULL, NULL, col_3 FROM table2 
WHERE col_3 in (SELECT DISTINCT col_1 FROM table1)

Despite the initial reaction to use a join, you are actually stacking these tables/result-sets on top of each other, therefore a UNION is more appropriate.