When joining two tables side-by-side you need to identify which row(s) in one correspond to which row(s) in the other. Conceptually, this is done by looking at each row in the first table, somehow identifying in the second table which row "refers to the same thing", and putting a new row in the joined table which consists of all the fields of the row in the first table, followed by all the fields of its matched row in the second table. The resulting table then has a number of columns equal to the sum of the number of columns in both input tables.
In practice, there are a number of complications. For one thing, each row in one table may be matched by zero, one or many rows in the the other. For another, defining what is meant by "referring to the same thing" may not be straightforward. There is also the problem of actually identifying these matches in a relatively efficient way (without explicitly comparing each row in one table with each row in the other, which would be far too slow for large tables).
A common example is the case of matching two object catalogues - suppose we have the following catalogues:
Xpos Ypos Vmag ---- ---- ---- 1134.822 599.247 13.8 659.68 1046.874 17.2 909.613 543.293 9.3and
x y Bmag - - ---- 909.523 543.800 10.1 1832.114 409.567 12.3 1135.201 600.100 14.6 702.622 1004.972 19.0and we wish to combine them to create one new catalogue with a row for each object which appears in both tables. To do this, you have to specify what counts as a match - in this case let's say that a row in one table matches (refers to the same object as) a row in the other if the distance between the positions indicated by their X and Y coordinates matches to within one unit (sqrt((Xpos-x)2 + (Ypos-y)2)<=1)). Then the catalogue we will end up with is:
Xpos Ypos Vmag x y Bmag ---- ---- ---- - - ---- 1134.822 599.247 13.8 1135.201 600.100 14.6 909.613 543.293 9.3 909.523 543.800 10.1There are a number of variations on this however - your match criteria might involve sky coordinates instead of Cartesian ones (or not be physical coordinates at all), you might want to match more than two tables, you might want to identify groups of matching objects in a single table, you might want the output to include rows which don't match as well...
The Match Window allows you to specify
To match two tables, use the Pair Match () button in the Control Window; to match more tables than two at once, use the other options on the Control Window's Join menu.