Chapter 6 SQL Data Manipulation Objectives of SQL

Chapter 6 SQL: Data Manipulation

Objectives of SQL u Database language should allow user to: – create database and relation structures – perform insertion, modification, deletion of data – perform simple and complex queries u Must perform tasks with minimal user effort u Command structure/syntax must be easy to learn u Portable 2

Objectives of SQL u SQL - transform-oriented language with 2 major components: – DDL for defining database structure – DML for retrieving and updating data 3

Objectives of SQL u SQL is relatively easy to learn: – Non-procedural » Specify what information you require, rather than how to get it; – Essentially free-format 4

Objectives of SQL u Consists of standard English words: 1) CREATE TABLE Staff(staff. No VARCHAR(5), l. Name VARCHAR(15), salary DECIMAL(7, 2)); 2) INSERT INTO Staff VALUES (‘SG 16’, ‘Brown’, 8300); 3) SELECT staff. No, l. Name, salary FROM Staff WHERE salary > 10000; 5

Objectives of SQL u Can be used by range of users – DBAs, management, application developers, and other types of end users u ISO standard now exists for SQL – Making it both the formal and de facto standard language for relational databases 6

History of SQL u 1974, D. Chamberlin (IBM San Jose Laboratory) defined language called ‘Structured English Query Language’ (SEQUEL) u 1976, revised version, SEQUEL/2 defined – Name subsequently changed to SQL for legal reasons 7

History of SQL u Still pronounced ‘see-quel’, though official pronunciation is ‘S-Q-L’ u IBM produced a prototype DBMS called System R, based on SEQUEL/2. u Roots of SQL – In SQUARE (Specifying Queries as Relational Expressions), which predates System R project 8

History of SQL u u u Late 70 s, ORACLE appeared; probably first commercial RDBMS based on SQL 1987, ANSI and ISO published initial standard for SQL 1989, ISO published addendum that defined ‘Integrity Enhancement Feature’ 1992, first major revision to ISO standard occurred, referred to as SQL 2 or SQL/92 1999, SQL: 1999 released with support for object-oriented data management Late 2003, SQL: 2003 released 9

Importance of SQL u SQL has become part of application architectures – IBM’s Systems Application Architecture u Strategic choice of many large and influential organizations (e. g. X/OPEN) u Federal Information Processing Standard (FIPS) to which conformance required for all sales of databases to American Government 10

Importance of SQL used in other standards and influences development of other standards as definitional tool: – ISO’s Information Resource Directory System (IRDS) Standard – Remote Data Access (RDA) Standard 11

Writing SQL Commands u SQL statement consists of: – reserved words – user-defined words – Reserved words – fixed part of SQL – Must be spelt exactly as required – Cannot be split across lines User-defined words – Made up by user – Represents names of database objects e. g. relations, columns, views – 12

Writing SQL Commands u u Most components of SQL statement - case insensitive – Except for literal character data More readable with indentation and lineation: – Each clause should begin on new line – Start of clause should line up with start of other clauses – If clause has several parts, each should appear on separate line and be indented under start of clause 13

Writing SQL Commands u Use extended BNF notation: - Upper-case letters represent reserved words - Lower-case letters represent user-defined words - | indicates choice among alternatives - Curly braces indicate required element - Square brackets indicate optional element. - … indicates optional repetition (0 or more) 14

Literals u Literals are constants used in SQL statements u All non-numeric literals must be enclosed in single quotes (e. g. ‘London’) u All numeric literals must not be enclosed in quotes (e. g. 650. 00) 15

SELECT Statement SELECT [DISTINCT | ALL] {* | [column. Expression [AS new. Name]] [, . . . ] } FROM Table. Name [alias] [, . . . ] [WHERE condition] [GROUP BY column. List] [HAVING condition] [ORDER BY column. List] 16

SELECT Statement FROM WHERE GROUP BY HAVING SELECT ORDER BY Specifies table(s) to be used Filters rows Forms groups of rows with same column value Filters groups subject to some condition Specifies which columns are to appear in output Specifies order of output 17

SELECT Statement u Order u Only of clauses cannot be changed SELECT and FROM are mandatory 18

Example 6. 1 All Columns, All Rows List full details of all staff. SELECT staff. No, f. Name, l. Name, address, position, sex, DOB, salary, branch. No FROM Staff; u Can use * as an abbreviation for ‘all columns’: SELECT * FROM Staff; 19

Example 6. 1 All Columns, All Rows 20

Example 6. 2 Specific Columns, All Rows Produce a list of salaries for all staff, showing only staff number, first and last names, and salary. SELECT staff. No, f. Name, l. Name, salary FROM Staff; 21

Example 6. 2 Specific Columns, All Rows 22

Example 6. 3 Use of DISTINCT List the property numbers of all properties that have been viewed. SELECT property. No FROM Viewing; 23

Example 6. 3 Use of DISTINCT u Use DISTINCT to eliminate duplicates: SELECT DISTINCT property. No FROM Viewing; 24

Example 6. 4 Calculated Fields Produce list of monthly salaries for all staff, showing staff number, first/last name, and salary. SELECT staff. No, f. Name, l. Name, salary/12 FROM Staff; 25

Example 6. 4 Calculated Fields u To name column, use AS clause: SELECT staff. No, f. Name, l. Name, salary/12 AS monthly. Salary FROM Staff; 26

Example 6. 5 Comparison Search Condition List all staff with a salary greater than 10, 000. SELECT staff. No, f. Name, l. Name, position, salary FROM Staff WHERE salary > 10000; 27

Example 6. 6 Compound Comparison Search Condition List addresses of all branch offices in London or Glasgow. SELECT * FROM Branch WHERE city = ‘London’ OR city = ‘Glasgow’; 28

Example 6. 7 Range Search Condition List all staff with a salary between 20, 000 and 30, 000. SELECT staff. No, f. Name, l. Name, position, salary FROM Staff WHERE salary BETWEEN 20000 AND 30000; u BETWEEN test includes endpoints of range 29

Example 6. 7 Range Search Condition 30

Example 6. 7 Range Search Condition u Negated version - NOT BETWEEN u BETWEEN does not add much to SQL’s expressive power. Could also write: SELECT staff. No, f. Name, l. Name, position, salary FROM Staff WHERE salary>=20000 AND salary <= 30000; u Useful for range of values 31

Example 6. 8 Set Membership List all managers and supervisors. SELECT staff. No, f. Name, l. Name, position FROM Staff WHERE position IN (‘Manager’, ‘Supervisor’); 32

Example 6. 8 Set Membership u. Negated version (NOT IN) u IN does not add much to SQL’s expressive power. Could have expressed this as: SELECT staff. No, f. Name, l. Name, position FROM Staff WHERE position=‘Manager’ OR position=‘Supervisor’; u IN more efficient when set contains many values 33

Example 6. 9 Pattern Matching Find all owners with the string ‘Glasgow’ in their address. SELECT owner. No, f. Name, l. Name, address, tel. No FROM Private. Owner WHERE address LIKE ‘%Glasgow%’; 34

Example 6. 9 Pattern Matching u SQL has two special pattern matching symbols: – %: sequence of zero or more characters – _ (underscore): any single character u LIKE ‘%Glasgow%’ means sequence characters of any length containing ‘Glasgow’ of 35

Example 6. 10 NULL Search Condition List details of all viewings on property PG 4 where a comment has not been supplied. u There are 2 viewings for property PG 4, one with and one without a comment. u Have to test for null explicitly using special keyword IS NULL: SELECT client. No, view. Date FROM Viewing WHERE property. No = ‘PG 4’ AND comment IS NULL; 36

Example 6. 10 NULL Search Condition u Negated version (IS NOT NULL) can test for non-null values 37

Example 6. 11 Single Column Ordering List salaries for all staff, arranged in descending order of salary. SELECT staff. No, f. Name, l. Name, salary FROM Staff ORDER BY salary DESC; 38

Example 6. 11 Single Column Ordering 39

Example 6. 12 Multiple Column Ordering Produce abbreviated list of properties in order of property type. SELECT property. No, type, rooms, rent FROM Property. For. Rent ORDER BY type; 40

Example 6. 12 Multiple Column Ordering 41

Example 6. 12 Multiple Column Ordering u Four flats in this list - as no minor sort key specified, system arranges these rows in any order it chooses u To arrange in order of rent, specify minor order: SELECT property. No, type, rooms, rent FROM Property. For. Rent ORDER BY type, rent DESC; 42

Example 6. 12 Multiple Column Ordering 43

SELECT Statement - Aggregates u ISO standard defines five aggregate functions: COUNT - returns number of values in specified column SUM - returns sum of values in specified column AVG - returns average of values in specified column MIN - returns smallest value in specified column MAX - returns largest value in specified column 44

SELECT Statement - Aggregates u Each operates on single column of table and returns single value u COUNT, MIN, and MAX apply to numeric and non-numeric fields – SUM and AVG used on numeric fields only u Each function eliminates nulls first and operates only on remaining non-null values – Except COUNT 45

SELECT Statement - Aggregates u COUNT(*) counts all rows of table – Includes nulls and duplicate values u Can use DISTINCT before column name to eliminate duplicates u DISTINCT has no effect with MIN/MAX – Has effect with SUM/AVG 46

SELECT Statement - Aggregates u Aggregate functions used only in SELECT list and HAVING clause u If SELECT list includes an aggregate function and there is no GROUP BY clause, SELECT list cannot reference column out with aggregate function Illegal: u SELECT staff. No, COUNT(salary) FROM Staff; 47

Example 6. 13 Use of COUNT(*) How many properties cost more than £ 350 per month to rent? SELECT COUNT(*) AS my. Count FROM Property. For. Rent WHERE rent > 350; 48

Example 6. 14 Use of COUNT(DISTINCT) How many different properties viewed in May ‘ 04? SELECT COUNT(DISTINCT property. No) AS my. Count FROM Viewing WHERE view. Date BETWEEN ‘ 1 -May-04’ AND ‘ 31 -May-04’; 49

Example 6. 15 Use of COUNT and SUM Find number of Managers and sum of their salaries. SELECT COUNT(staff. No) AS my. Count, SUM(salary) AS my. Sum FROM Staff WHERE position = ‘Manager’; 50

Example 6. 16 Use of MIN, MAX, AVG Find minimum, maximum, and average staff salary. SELECT MIN(salary) AS my. Min, MAX(salary) AS my. Max, AVG(salary) AS my. Avg FROM Staff; 51

SELECT Statement - Grouping u Use GROUP BY clause to get sub-totals u SELECT and GROUP BY closely integrated: – Each item in SELECT list must be singlevalued per group – SELECT clause may only contain: » » column names aggregate functions constants expression involving combinations of above 52

SELECT Statement - Grouping u All column names in SELECT list must appear in GROUP BY clause unless name used only in aggregate function u If WHERE used with GROUP BY: – WHERE applied first – Then groups formed from remaining rows satisfying predicate u ISO considers two nulls to be equal for purposes of GROUP BY 53

Example 6. 17 Use of GROUP BY Find number of staff in each branch and their total salaries. SELECT branch. No, COUNT(staff. No) AS my. Count, SUM(salary) AS my. Sum FROM Staff GROUP BY branch. No ORDER BY branch. No; 54

Example 6. 17 Use of GROUP BY 55

Restricted Groupings – HAVING clause u HAVING clause designed for use with GROUP BY to restrict groups that appear in final result table u Similar to WHERE: – WHERE filters individual rows – HAVING filters groups u Column names in HAVING clause must appear in GROUP BY list or be contained within aggregate function 56

Example 6. 18 Use of HAVING For each branch with more than 1 member of staff, find number of staff in each branch and sum of their salaries. SELECT branch. No, COUNT(staff. No) AS my. Count, SUM(salary) AS my. Sum FROM Staff GROUP BY branch. No HAVING COUNT(staff. No) > 1 ORDER BY branch. No; 57

Example 6. 18 Use of HAVING 58

Subqueries u Some SQL statements can have SELECT embedded within them u Ssubselect can be used in WHERE and HAVING clauses of an outer SELECT – Called subquery or nested query u Subselects may also appear in UPDATE, and DELETE statements INSERT, 59

Example 6. 19 Subquery with Equality List staff who work in branch at ‘ 163 Main St’. SELECT staff. No, f. Name, l. Name, position FROM Staff WHERE branch. No = (SELECT branch. No FROM Branch WHERE street = ‘ 163 Main St’); 60

Example 6. 19 Subquery with Equality u Inner SELECT finds branch number for branch at ‘ 163 Main St’ (‘B 003’). u Outer SELECT then retrieves details of all staff who work at this branch. u Outer SELECT then becomes: SELECT staff. No, f. Name, l. Name, position FROM Staff WHERE branch. No = ‘B 003’; 61

Example 6. 19 Subquery with Equality 62

Example 6. 20 Subquery with Aggregate List all staff whose salary is greater than the average salary, and show by how much. SELECT staff. No, f. Name, l. Name, position, salary – (SELECT AVG(salary) FROM Staff) As Sal. Diff FROM Staff WHERE salary > (SELECT AVG(salary) FROM Staff); 63

Example 6. 20 Subquery with Aggregate u Cannot write ‘WHERE salary > AVG(salary)’ u Instead, use subquery to find average salary (17000), and then use outer SELECT to find those staff with salary greater than this: SELECT staff. No, f. Name, l. Name, position, salary – 17000 As sal. Diff FROM Staff WHERE salary > 17000; 64

Example 6. 20 Subquery with Aggregate 65

Subquery Rules u ORDER BY clause may not be used in subquery – May be used in outermost SELECT u Subquery SELECT list must consist of single column name or expression – Except for subqueries that use EXISTS u By default, column names refer to table name in FROM clause of subquery u Can refer to table in FROM using alias 66

Subquery Rules u When subquery is operand in comparison – Subquery must appear on right-hand side u Subquery may not be used as operand in an expression 67

Example 6. 21 Nested subquery: use of IN List properties handled by staff at ‘ 163 Main St’. SELECT property. No, street, city, postcode, type, rooms, rent FROM Property. For. Rent WHERE staff. No IN (SELECT staff. No FROM Staff WHERE branch. No = (SELECT branch. No FROM Branch WHERE street = ‘ 163 Main St’)); 68