CP363 Lecture Notes - Lecture 14: Uncorrelated Random Variables
8 Apr 2019
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Nested queries, joined tables and outer joins (in the from clause) Each individual null value considered to be different from every other null value. Sql allows queries that check whether an attribute value is null. Complete select-from-where blocks within where clause of another query. Compares value v with a set (or multiset) of values v. Evaluates to true if v is one of the elements of v. Correlated if inner query references attribute in outer query in where clause. Uncorrelated query compute only once (independant inner queries) Correlated have to be recomputed over and over again because attributes depends on outer query. Retrieve all employee sins that work the same number of hours as one employee (slide. Use other comparison operators to compare a single value v. Returns true if the value v is equal to some value in the set v nad is hence equivalent to in. Other operators can be combined with any ( or some); >,
