Question

I have the following code:

get_list_a()
{
    $MYSQL -B -u $USER --passwword="$PW" $DB1 <<EOF
select name, value from mytable_a
EOF
}
get_list_b()
{
    $MYSQL -B -u $USER --passwword="$PW" $DB2 <<EOF
select name, value from mytable_b
EOF
}

get_list_a >$test.txt

Now I need to combine a and b first and remove all dups(key is name, the first column) and then write them to test.txt. List a and list b by itself are assumed to be distinct. If x in a and y in b exist such that x.name=y.name, then I only want to keep x.
how do I do it? note: merging in SQL is not an option since they are in different db with different collation.

An example:
get_list_a prints

aaa bbb
ccc ddd

get_list_b prints

aaa fff
ggg hhh

I want the following to be written to the file:

aaa bbb
ccc ddd
ggg hhh

Answer 1

Would a SQL query along these lines work? (Untested)

SELECT COALESCE(x.name,y.name),COALESCE(x.value,y.value)
FROM mytable_a AS x
FULL JOIN mytable_b AS y
ON x.name = y.name;

Edit: OK, if they’re in separate DBs, and the fields are space-separated as you indicate in a comment, I would probably use associative arrays in perl or awk, letting the values from x (a) overwrite the values from y (b). Something like this (still untested):

get_list_a > x.txt
get_list_b > y.txt
cat y.txt x.txt | awk '{ data[$1] = $2; } END { for (i in data) { print i, data[i]; }}'