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Session 3

Session 3. Welcome : To session 3-the second learning sequence “ Relational algebra “ Recap : In the previous learning sequences, we discussed the relational model. Present learning : We shall explore the following topics: - Relational algebra.

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Session 3

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  1. Session 3 Welcome: To session 3-the second learning sequence “ Relational algebra “ Recap : In the previous learning sequences, we discussed the relational model. Present learning: We shall explore the following topics: - Relational algebra. - Some operators.

  2. Relational Algebra

  3. Query Languages • Language in which user requests information from the database. • Categories of languages • procedural • non-procedural • “Pure” languages: • Relational Algebra • Tuple Relational Calculus • Domain Relational Calculus • Pure languages form underlying basis of query languages that people use.

  4. Relational Algebra • Procedural language • Four basic operators • select • project • union • Intersection • The operators take one or more relations as inputs and give a new relation as a result.

  5. Select Operation Notation: p(r) p is called the selection predicate Defined as: p(r) = {t | t  rand p(t)} Where p is a formula in propositional calculus consisting of terms connected by :  (and),  (or),  (not)Each term is one of: <attribute> op <attribute> or <constant> where op is one of: =, , >, . <.  Example of selection:branch-name=“Perryridge” (account)

  6. Select Operation – Example Account

  7. Select Operation – Example The result is the relation: 7

  8. Select Operation – Example Balance >“700” (account) 8

  9. Project Operation Notation:A1, A2, …, Ak (r) where A1, A2 are attribute names and r is a relation name. The result is defined as the relation of k columns obtained by erasing the columns that are not listed Duplicate rows removed from result, since relations are sets

  10. Project Operation E.g. To eliminate the branch-name attribute of accountaccount-number, balance (account) The result relation is:

  11. Project Operation 11

  12. Union Operation • Notation: r s • Defined as: r s = {t | t  r or t  s} • For r s to be valid. 1. r,s must have the same arity (same number of attributes) 2. The attribute domains must be compatible (e.g., 2nd column of r deals with the same type of values as does the 2nd column of s)

  13. Union Operation E.g. to find all customers with either an account or a loancustomer-name (depositor)  customer-name (borrower) depositor borrower

  14. Union Operation • customer-name (depositor) customer-name (borrower)

  15. Union Operation • The result relation is:

  16. Intersection Operation • Notation: r s • Defined as: • rs ={ t | trandts } • Assume: • r, s have the same arity • attributes of r and s are compatible • Note: rs = r - (r - s)

  17. Intersection Operation E.g. to find all customers with an account and a loancustomer-name (depositor)  customer-name (borrower) depositor borrower

  18. Intersection Operation • customer-name (depositor) customer-name (borrower)

  19. Intersection Operation The result relation is:

  20. Relational Algebra Summary: In this learning sequence, we discussed Four basic operators of the topic relational algebra.

  21. END

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