Here are some sample completions:
A schema is to a program as a pattern is to a dress.
Why? A pattern gives a seamstress a guide or template to use
when creating a dress. A schema gives a programmer a guide or template
to complete a program.
A schema is to a program as an outline is to a paper.
Why? An outline gives an overall structure to a paper but leaves
out the details. A schema gives an overall structure to a piece of
a program while the programmer is left to fill in the details.