Free Keys, ER and normalization lessonsKeys, ER and normalization · Stage 0 · Relations and keys · Step 2
Relational algebra
Selection, projection, join, union and the other operators that SQL is built on, and how to read an algebra expression. About 13 minutes.
Which SQL does π_{city}(σ_{age > 30}(person)) correspond to?
- ASELECT DISTINCT city FROM person WHERE age > 30
- BSELECT city FROM person GROUP BY age HAVING age > 30
- CSELECT city FROM person JOIN person ON age > 30
Show the answer
SELECT DISTINCT city FROM person WHERE age > 30. Selection is WHERE, projection is SELECT, and projection removes duplicates.
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