SQL Overview cont d. SQL Overview. ICOM 5016 Database Systems. SQL Data Types. A language higher-level than general purpose languages

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1 SQL Overview ICOM 5016 Database Systems Structured Query Language Amir H. Chinaei Department of Electrical and Computer Engineering University of Puerto Rico, Mayagüez Slides are adapted from: Database System Concepts, 5th Ed. Silberschatz, Korth and Sudarshan See for conditions on re-use A language higher-level than general purpose languages Programmers avoid many details Simple enough to be optimized very well Efficient query execution Developed in IBM (SQUEL-XRM and System R) Implementation of R.A. Set/Bag semantics and operations Aggregation It s not case-sensitive, except in quoted strings. 2 SQL Overview cont d Major parts of the language: DML (Data Manipulation Language) Query language Update language DDL (Data Definition Language) defines schema for relations creates (modifies/destroys) database objects. DCL (Data Control Language) access control Also Embedded and Dynamic SQL necessary for application development Triggers Client-Server Execution, etc. Values of attributes in SQL: SQL Data Types Integer integer (32 bit) Smallint integer (16 bit) Decimal (m,n) fixed decimal float IEEE float (32 bit) char(n) character string (length n) varchar(n) variable length string (at most n) date year/month/day time hh:mm:ss.ss 3 4 1

2 Roadmap to SQL Queries? the select block simple select-from-where set operations subqueries in the from clause grouping, aggregation, and having clauses duplicates and distinct subqueries in the where clause ordering the output Bag operations naming queries and views Our Running Example Food (Name, Cuisine,Type) Foodservice (Name, Category, Rate) Branch (Name, B#, Location, Since) Student (S#, Name, Major, Hobby) Likes (Student, Food) Provides (Foodservice, Food, Price) Frequents (Student, Name, B#) 5 6 The form of a basic SQL query: Select-From-Where Example: S-F-W List all CS student names and their hobby: SELECT some attributes FROM some relations WHERE some conditions on relations instance db2 >> select name, hobby from student where major= CS ; Name Hobby Natasha Tennis Eric Chess Elena Dance from student where major= CS ;

3 Conceptual Evaluation Strategy Naming Attributes Variety of execution plans: e.g. Open the FROM clause relation Start from the first tuple While not end of the relation Check if the tuple meets the WHERE clause condition If yes, apply the SELECT clause projection S# Name Major Hobby Natasha CS Tennis 08 Scott Eng Chess 87 Mahsa Math Dance 45 Eric CS Chess 93 Ali Math Music 32 Elena CS Dance What if two relations use the same attribute name? Foodservice (Name,...) Branch (Name,...) and we want to get, e.g., locations of some foodservices Like in R.A., we prefix the ambiguous attributes names by the name of the appropriate relation 9 10 Example List locations of fast food services. db2 >> select location, foodservice where category= Fast Food and branch.name=foodservice.name; Location SLC DC CEIT South Campus Naming Relations what if we need to use the same table to be used several times in the FROM clause? list cuisines with at least two foods use corelation names in the FROM clause e.g., FROM food f1, food f

4 Example list cuisines with at least two foods db2 >> select cuisine... Summary of the "FROM" Clause Properties Syntax: FROM R 1 [n 1 ],, R k [n k ] R i are relation (table) names n i are distinct identifiers (corelations) the clause represents a product R 1 R k all attributes of R i s are distinct we use (co)relation names to resolve ambiguities can NOT appear alone only as a part of the SELECT block Using Expressions Idea. to create values in the answer tuples using built-in functions: on numeric types: +, -, *, /, (usual arithmetic) on strings: (concatenation), substr,... constants (of appropriate types) "SELECT 1" is a valid query in SQL/92 UDF (user defined functions) Naming the Results Results of queries are tables What are the names of attributes in the result of a SELECT clause? A single attribute: inherits the name An expression: implementation dependent we can and should explicitly name the resulting attributes: Syntax: expr AS id where id is the new name Note: all attribute names MUST be present in the relations of the FROM clause

5 Renaming Attributes To demonstrate attribute names differently To resolve name conflicts Example: Expressions List provided foods in Canadian dollar (assume $1.00=CND$ 1.1) use AS newname to rename an attribute Example: db2 >>... Customer Food Natasha Poutine Scott Fish Burger Mahsa Kabab Eric Sushi Ali Chicken Elena Poutine db2 >> select.. from provides; Foodservice Food CND$ Brubaker s Pizza 5.50 Math C&D Sushi 6.40 Bon Appetite Sushi List all veggie foods. Example: Constants db2 >> select name, cuisine, veggie as type from food where type= No-Meat ; Name Cuisine Type Mussaca Greek veggie Sushi Japanese veggie Eggplants Chinese veggie Mixed rice Chinese veggie Summary of the "SELECT" Clause Syntax: SELECT [DISTINCT] e 1 [ AS i 1 ],,e k [ AS i k ] Properties eliminate superfluous attributes from answers form expressions: built-in functions applied to values of attributes give names to attributes in the answer

6 More on the "WHERE" Clause conditions on tuples that qualify for the answer WHERE C standard atomic conditions: equality: =,!= order: <, <=, >, >=, <> conditions may involve expressions Similar to expressions in the SELECT clause Example: where List all new foodservices (started this year) where since >= 2010; Name B# Location Since Tim Horton s 6 RCH 2010 Browsers 2 DC Boolean Connectives Atomic conditions can be combined using boolean connectives: AND (conjunction) OR (disjunction) NOT (negation) If necessary, use parentheses in the usual way. Example: Connectives List all new RCH foodservices (started this year) where since >= 2010 and location=`rch`;

7 Quotes in Strings Pattern Matching List locations of all Tim Horton s. db2 >> select location where name=`tim Horton``s`; Location SLC DC South Campus Modern Language Conditions in WHERE clauses can compare a string with a pattern Syntax: <Attribute> [NOT] LIKE <pattern> Pattern wild-card symbols % any string _ any character Example: Pattern matching Find all foodservices located somewhere in RCH where.. Name B# Location Since Tim Horton s 6 RCH 2010 Browsers 3 RCH 2nd floor 2007 Hot Hot Dog 1 by RCH 2007 Null Values Null is a value that SQL considers for those attributes of a tuple for which a value has not been provided yet Null can be interpreted in at least two ways: Unavailable: e.g., we know Browsers opened some time ago, but its opening year is not provided in the database. Inapplicable: e.g., the value of attribute cuisine is not applicable to bottled water

8 Semantics of Null Example 1: null Is defined in a 3-value logic: TRUE, FALSE, UNKNOWN A null value is not comparable to any non-null value If a null value is compared to a non-null value, the result is always null (UNKNOWN). But a tuple satisfies a condition only if the comparison result is TRUE None of the standard atomic conditions are applicable to null To check if attribute a i value is null, use: a i IS [NOT] NULL Assume the following instance for the Foodservice relation Foodservice Then, what do you think about the following query? from foodservice where rate>5 or rate<=5; Name Category Rate Browsers Tim Horton s Brubaker s Coffee shop Coffee shop Cafe Example 2: null List all foodservices whose rating is unavailable. db2 >> select name, category from foodservice where.; Name Category Tim Horton s Coffee shop Set Operations at Glance Note. Answers to Select Blocks are relations (sets of tuples), we can apply set operations on them: set union: (Q1) UNION (Q2) the set of tuples in Q1 or in Q2 used to express or set difference: (Q1) EXCEPT (Q2) the set of tuples in Q1 but not in Q2 used to express and not set intersection: (Q1) INTERSECT (Q2) the set of tuples in both Q1 and Q2 used to express and (redundant, rarely used) Note: Q1 and Q2 must have union-compatible signatures: same number and types of attributes

9 Example: Set Union List all foodservices which have a rating over 7 or they have a branch somewhere in RCH. db2 >> (select name from foodservice where rate >7) union (select name where location like `%RCH% ); How about OR instead of UNION? A common mistake: use of OR in the WHERE clause instead of the UNION operator! An incorrect solution: db2 >> select name from foodservice f, branch b where f.name = b.name and (rate >7 or location like `%RCH% ); often works; but imagine a foodservice that does not have any branch! Example: Set Difference List all foodservices which have a rating over 7 and they do not have a branch in RCH. db2 >> (select name from foodservice where rate >7) except (select name where location =`RCH ;) Recommended Exercises: Ramakrishnan s