# what does “\” symbol mean? [closed]

I am new to database learning and came across this `\` symbol. I googled but couldn't find what it meant.

It was used in the context `Q = R\S`, where Q, R, and S are relations.

(The = sign here means "is equivalent to").

## closed as unclear what you're asking by Max Vernon, James Anderson, Tom V, billinkc, Andriy MMay 3 '16 at 11:47

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The `\` symbol is often used in mathematics for set difference, besides dash or minus symbol (`-`)
Since tables and relations are very similar to sets and the operations of union, intersection and difference are similarly defined (see Wikipedia: Relational algebra, set operators), the `\` symbol is also sometimes used for difference.
If the `Q = R\S` expression is found in a database theory book and `Q`, `R` and `S` are relations, it means that `Q` is a relation (table) that has the same attributes (columns) as `R` and `S` and also has those tuples (rows) that are in relation `R` but not in relation `S`.