tailieunhanh - Chapter 3: More SQL

Chapter 3: More SQL includes More Complex SQL Retrieval Queries, Specifying Constraints as Assertions and Actions as Triggers, Views in SQL, Schema Change Statements in SQL, Database Programming and something else. | Chapter 3 MORE SQL Copyright 2004 Pearson Education Inc. Outline More Complex SQL Retrieval Queries Specifying Constraints as Assertions and Actions as Triggers Views in SQL Schema Change Statements in SQL Database Programming Elmasri Navathe Fundamentals of Database Systems Fourth Edition Copyright 2004 Ramez Elmasri and Shamkant Navathe Slide 3 - 2 NULLS IN SQL QUERIES SQL allows queries that check if a value is NULL missing or undefined or not applicable SQL uses IS or IS NOT to compare NULLs because it considers each NULL value distinct from other NULL values so equality comparison is not appropriate . Query 14 Retrieve the names of all employees who do not have supervisors. Q14 SELECT FNAME LNAME FROM EMPLOYEE WHERE SUPERSSN IS NULL Note If a join condition is specified tuples with NULL values for the join attributes are not included in the result__________________________________________________ Elmasri Navathe Fundamentals of Database Systems Fourth Edition Slide 3 - 3 Copyright 2004 Ramez Elmasri and Shamkant .