Get lines from file that match strings in another file using AWK - csv

Question

I have file named key and another csv file named val.csv. As you can imagine, the file named key looks something like this:
123
012
456
The file named val.csv has multiple columns and corresponding values. It looks like this:
V1,V2,V3,KEY,V5,V6
1,2,3,012,X,t
9,0,0,452,K,p
1,2,2,000,L,x
I would like get the subset of lines from val.csv whose value in the KEY column matches the values in the KEY file. Using the above example, I would like to get an output like this:
V1,V2,V3,KEY,V5,V6
1,2,3,012,X,t
Obviously these are just toy examples. The real KEY file I am using has nearly 500,000 'keys' and the val.csv file has close to 5 million lines in them. Thanks.

Answer 1

$ awk -F, 'FNR==NR{k[$1]=1;next;} FNR==1 || k[$4]' key val.csv
V1,V2,V3,KEY,V5,V6
1,2,3,012,X,t
How it works
FNR==NR { k[$1]=1;next; }
This saves the values of all keys read from the first file, key.
The condition is FNR==NR. FNR is the number of lines read so far from the current file and NR is the total number of lines read. Thus, if FNR==NR, we are still reading the first file.
When reading the first file, key, this saves the value of key in associative array k. This then skips the rest of the commands and starts over on the next line.
FNR==1 || k[$4]
If we get here, we are working on the second file.
This condition is true either for the first line of the file, FNR==1, or for lines whose fourth field is in array k. If the condition is true, awk performs the default action which is to print the line.