SQL Questions et reponses d'entretien
Question 46. Minus
Minus is the product of two tables listing only the non-matching rows.
Question 47. Correlated Subquery
Correlated Subquery is a subquery that is evaluated once for each row processed by the parent statement. Parent statement can be Select, Update or Delete. Use CRSQ to answer multipart questions whose answer depends on the value in each row processed by parent statement.
Question 48. Multiple columns
Multiple columns can be returned from a Nested Subquery.
Question 49. Sequences
Sequences are used for generating sequence numbers without any overhead of locking. Drawback is that after generating a sequence number if the transaction is rolled back, then that sequence number is lost.
Question 50. Synonyms
Synonyms is the alias name for table, views, sequences & procedures and are created for reasons of Security and Convenience.
Two levels are Public - created by DBA & accessible to all the users. Private - Accessible to creator only. Advantages are referencing without specifying the owner and Flexibility to customize a more meaningful naming convention.
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