hding is right. In that case, declare a multicolumn unique index. That does precisely what you are asking for. MySQL will look at the first column first, so it should be the more unique. [OB] A table needs a primary key.

create table depends
        (uses varchar(80) not null,
        usedby varchar(80) not null,
        unique deps (usedby,uses));
# naming the index is unnecessary, but may be a convenience
[download]

Btw, why not represent a leaf node by usedby=NULL? That would give a faster select for leaf/branch.

After Compline,
Zaxo