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Description
Flashcards by nurhidayah.mazni, updated more than 1 year ago
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Created by nurhidayah.mazni
over 8 years ago
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| Question | Answer |
| Basic of "RELATIONAL ALGEBRA" Operations | |
| Projection | * Retains only attributes that are in the “projection list". *The 1st query will produce a relation with only one column which is age. While the 2nd query produces a relation with two columns: sname and rating. * The number of rows depends on the number of unique content. e.g. 1st query, 2 rows, 2nd query, 4 rows. # Projection operator eliminates duplicates rows . |
| Selection | # Selects rows that satisfy selection condition. Can also have multiple conditions. # Result is a relation with same number of columns BUT probably lesser number of rows. |
| Union and Set-Difference | All of these operations take two input relations, which must be union compatible: - Same number of fields. - Corresponding’ fields have the same type. |
| Union | |
| Set Difference | |
| Cross-Product | Result schema has one field per field of R1 and R2, with field names `inherited’ if possible. May have a naming conflict: Both R1 and R2 have a field with the same name. |
| Compound Operator: Intersection | Intersection takes two input relations, which must be union-compatible.. Q: How to express it using basic operators? |
| Intersection | |
| Compound Operator: Join | Joins are compound operators involving cross product, selection, and (sometimes) projection. Most common type of join is a “natural join” (often just called “join”). R S conceptually is: Compute R X S Select rows where attributes that appear in both relations have equal values and domains. Project all unique atttributes and one copy of each of the common ones. |
| Natural Join Example | |
| Outer Joins | Left outer Join: each row in R1 will appear in results even if no matching row of R2 (null). Right outer Join: each row in R2 will appear in results even if no matching row of R1 (null). Full outer Join: each row in R1 will appear in even if no matching row of R2 and each row of R2 will appear even if no matching row of R1. |
| Compound Operator: Division | Examples of Division A/B (AB) |
| Aggregate Functions | Aggregation function takes a collection of values and returns a single value as a result. avg: average value min: minimum value max: maximum value sum: sum of values count: number of values |
| Aggregate Operations | Aggregate operation in relational algebra. G1, G2, …, Gn ℱ F1( A1), F2( A2),…, Fn( An) (E) E is any relational-algebra expression. G1, G2 …, Gn is a list of attributes on which to group (can be empty). Each Fi is an aggregate function. Each Ai is an attribute name |
| Aggregate Operation – Example | |
| Assignment Operation | The assignment operation () provides a convenient way to express complex queries. *Write query as a sequential program consisting of a series of assignments followed by an expression whose value is displayed as a result of the query. *Assignment must always be made to a temporary relation variable. |
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