CHAPTER 4 MISSING INFORMATION ADVANCED DATABASE SYSTEMS. Assist. Prof. Dr. Volkan TUNALI

Transcription

1 CHAPTER 4 MISSING INFORMATION ADVANCED DATABASE SYSTEMS Assist. Prof. Dr. Volkan TUNALI

2 Topics 2 Introduction The 3VL Approach NULLs and Keys Outer Join SQL Facilities

3 Introduction 3 Information is often missing in the real world. Date of birth unknown Speaker to be announced Present address not known We need some way of dealing with such missing information inside database systems. An approach to this problem is usually based on nulls and three-valued logic (3VL).

4 Null Value 4 WEIGHT column of PART table on a row of P7 is NULL, what does it mean? We know that the part P7 exists We also know that it has a weight But we don t know its weight What do we do? We mark or flag the WEIGHT position in that row as null. Null is not a value! The row contains no WEIGHT value at all.

5 Scalar Operators with NULL 5 Any numeric expression evaluates to NULL if any of the operands of that expression is NULL. If WEIGHT is NULL then all the following expressions evalute to NULL: WEIGHT WEIGHT 454 WEIGHT * 454 WEIGHT / WEIGHT - WEIGHT WEIGHT WEIGHT is also NULL, not ZERO! String operators also work similarly: If CITY is NULL, then in + CITY expression also evaluates to NULL

6 Three-Valued Logic (3VL) 6 2VL True / False 3VL True / False / Unknown Any scalar comparison in which either of the comparands is null evaluates to the unknown truth value, instead of true or false. Yes, UNK is a value unlike NULL. We ll use UNK instead of unknown. UNK is not the same thing as NULL!

7 Comparisons with NULL 7 If A is NULL, then (A > B) evaluates to UNK regardless of the value of B (whether B is NULL or not). Two NULLs are not equal! If A and B are both NULL, then (A = B) evaluates to UNK, not true! They are not unequal, either. (A!= B) evaluates to UNK too.

8 Boolean Operators with UNK 8 AND TRUE UNK FALSE OR TRUE UNK FALSE NOT TRUE TRUE UNK FALSE TRUE TRUE TRUE TRUE TRUE FALSE UNK UNK UNK FALSE UNK TRUE UNK UNK UNK UNK FALSE FALSE FALSE FALSE FALSE TRUE UNK FALSE FALSE TRUE A=3, B=4, C=NULL A > B AND B > C : FALSE A > B OR B > C : UNK A < B OR B < C : TRUE NOT (A = C) : UNK

9 NULLs and Keys 9 Primary Keys Entity Integrity: No component of primary key of any base relvar (table) is allowed to accept nulls. Foreign Keys Referential Integrity (original version): The database must not contain unmatched foreign key values. Null is exception to this rule; a foreign key can be allowed to accepts nulls.

10 Outer Join 1 10 SELECT S.S#, S.SNAME, S.STATUS, S.CITY, SP.P#, SP.QTY FROM S INNER JOIN SP ON SP.S# = S.S#

11 Outer Join 2 11 SELECT S.S#, S.SNAME, S.STATUS, S.CITY, SP.P#, SP.QTY FROM S OUTER JOIN SP ON SP.S# = S.S#

12 Outer Join 3 12 Three types of Outer Join Left Outer Join: Preserves information from the first operand Right Outer Join: Preserves information from the second operand Full Outer Join: Preserves information from both operands

13 Outer Join Example 1 13 PART Table ORDER Table

14 Outer Join Example 2 14 SELECT O.ORDER_ID, O.ORDER_DATE, P.PART_CODE, P.PART_NAME FROM [ORDER] O INNER JOIN [PART] P ON P.PART_ID = O.PART_ID

15 Outer Join Example 3 15 SELECT O.ORDER_ID, O.ORDER_DATE, P.PART_CODE, P.PART_NAME FROM [ORDER] O LEFT OUTER JOIN [PART] P ON P.PART_ID = O.PART_ID

16 Outer Join Example 4 16 SELECT O.ORDER_ID, O.ORDER_DATE, P.PART_CODE, P.PART_NAME FROM [ORDER] O RIGHT OUTER JOIN [PART] P ON P.PART_ID = O.PART_ID

17 Outer Join Example 4 17 SELECT O.ORDER_ID, O.ORDER_DATE, P.PART_CODE, P.PART_NAME FROM [ORDER] O FULL OUTER JOIN [PART] P ON P.PART_ID = O.PART_ID

18 SQL Facilities 1 18 How to test NULL value? Wrong Way: SELECT * FROM [ORDER] WHERE PART_ID = NULL Correct Way: SELECT * FROM [ORDER] WHERE PART_ID IS NULL Remember: NULL is not equal to NULL! Also: WHERE PART_ID IS NOT NULL Use this approach for IF/WHILE statements in T-SQL IS [NOT] NULL

19 SQL Facilities 2 19 SQL Server provides functions for testing NULL. ISNULL(X, Y): If X is NULL then returns Y, otherwise returns X. COALESCE(X, Y,, Z): Returns value of first nonnull operand if any, otherwise returns null.

20 SQL Facilities 3 20 What about aggregate operators? COUNT, SUM, AVG, MIN, MAX? SELECT COUNT(PART_ID), SUM(PART_ID), AVG(PART_ID), MIN(PART_ID), MAX(PART_ID) FROM [ORDER]

21 SQL Facilities 4 21 What about aggregate operators on empty sets? SELECT COUNT(PART_ID), SUM(PART_ID), AVG(PART_ID), MIN(PART_ID), MAX(PART_ID) FROM [ORDER] WHERE 1 = 0