A duplicate happens when in a set two elements have an equivalence relationship that returns true.
relational operators can create duplicates but no all.
See below:
| Operator | Symbol | Creates Duplicates? |
|---|---|---|
| Selection | σ | No |
| Projection | π | Yes |
| Cross-product | × | No |
| Set-difference | − | No |
| Union | ∪ | Yes |
| Join | ⋈ | No |