CS2 Current Technologies Lecture 2: SQL Programming Basics

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1 T E H U N I V E R S I T Y O H F R G E D I N B U CS2 Current Technologies Lecture 2: SQL Programming Basics Dr Chris Walton (cdw@dcs.ed.ac.uk) 4 February 2002

2 The SQL Language 1 Structured Query Language (SQL) is used for relational database programming. SQL is the most commonly used programming language in the world! SQL is essentially divided into two languages: 1. A Data Definition Language (DDL). 2. A Data Manipulation Language (DML). We define the SQL query (part of the DML) in this lecture. queries obey the relational closure property: the input and result of every query is a table.

3 Relational Databases (Quick Review) 2 All data in the database are stored in tables. Each table has a name so that it can be identified in a query. Each column in the table is labelled by an attribute name. Rows do not have names or labels. Each row describes information about one particular entity. Example Database Table DEPT: DEPTNO DNAME LOC 10 ACCOUNTING NEW YORK 20 RESEARCH DALLAS 30 SALES CHICAGO 40 OPERATIONS BOSTON

4 Example Table 3 NO ENAME JOB MGR HIREDATE SAL COMM DEPTNO 7369 SMITH CLERK DEC ALLEN SALESMAN FEB WARD SALESMAN FEB JONES MANAGER APR MARTIN SALESMAN SEP BLAKE MANAGER MAY CLARK MANAGER JUN SCOTT ANALYST JUN KING PRESIDENT 17-NOV TURNER SALESMAN SEP ADAMS CLERK JUL JAMES CLERK DEC FORD ANALYST DEC MILLER CLERK JAN

5 Selecting Specific Rows and Columns 4 Example - Retrieve the salary for the person with employee number 7902: SAL NO = 7902; SAL 3000 Example - Retrieve the names of all CLERKs who work in department 20: ENAME DEPTNO = 20 AND JOB = CLERK ; ENAME SMITH ADAMS

6 SQL Conditionals 5 There are six basic conditionals in SQL: =, <> (or!=), <, <=, >, and>=. Example - Testing for a negative search condition: ENAME JOB = MANAGER AND DEPTNO!= 20; ENAME BLAKE CLARKE Example - Testing for a range: ENAME SAL > 2500; ENAME JONES BLAKE SCOTT KING FORD

7 Use of DISTINCT, BETWEEN, and NOT 6 Example - Removing duplicate entries: DISTINCT JOB SAL BETWEEN 1000 AND 2000; JOB SALESMAN CLERK Example - Negating a condition (alternative): DISTINCT JOB SAL NOT BETWEEN 1000 AND 4000; JOB CLERK PRESIDENT

8 Ordering the Rows of a Query (ORDER BY) 7 Example - List all employees in department 10 by their salary: NO, ENAME, HIREDATE, SAL DEPTNO = 10 ORDER BY SAL; NO ENAME HIREDATE SAL 7934 MILLER 23-JAN CLARK 09-JUN KING 17-NOV Example - All employees in order of their jobs and in descending salary order: ORDER BY JOB, SAL, ENAME JOB IN ( CLERK, MANAGER ) JOB, SAL DESC; JOB SAL ENAME CLERK 1300 MILLER CLERK 1100 ADAMS CLERK 950 JAMES CLERK 800 SMITH MANAGER 2975 JONES MANAGER 2850 BLAKE MANAGER 2450 CLARK

9 Arithmetic Functions in SQL 8 Arithmetic Functions are applied to individual values, e.g: ENAME, SAL+COMM JOB = SALESMAN ; ENAME SAL+COMM ALLEN 1900 WARD 1750 MARTIN 2650 TURNER 1500 The main SQL arithmetic functions are shown below: + Addition POWER Exponentiation Subtraction ROUND Rounding Multiplication TRUNC Truncation / Division ABS Absolute Value % Modulo SQRT Square Root

10 Handling NULL Values 9 Example - Employees in department 30 who do not earn commission: ENAME, SAL, COMM COMM IS NULL AND DEPTNO=30; ENAME SAL COMM BLAKE 2850 JAMES 950 Example - Employees in department who earn commission: ENAME, SAL, COMM COMM IS NOT NULL AND DEPTNO=30; ENAME SAL COMM ALLEN WARD MARTIN TURNER

11 Aggregate Functions in SQL 10 The five main aggregate functions are: SUM, AVG, MAX, MIN, and COUNT. These functions ignore NULL values. Aggregate functions are applied to sets of values, e.g: AVG(SAL) JOB = CLERK ; AVG Example - How many analysts are working for the company? COUNT(*) JOB = ANALYST ; COUNT 2

12 More Function Examples in SQL 11 Example - What are the minimum and maximum commissions paid to salesman, and what is the total amount of commission paid by the company? MIN(COMM), MAX(COMM), SUM(COMM), COUNT(COMM) JOB = SALESMAN ; MIN MAX SUM COUNT Example - MAX and MIN also work alphabetically: MIN(ENAME), MAX(ENAME) ; MIN ADAMS MAX WARD

13 Non-Numeric Functions 12 Example - Wild-card string matching: ENAME, JOB, DEPTNO ENAME LIKE M% ; ENAME JOB DEPTNO MARTIN SALESMAN 30 MILLER CLERK 10 Example - Working with dates: ENAME, HIREDATE HIREDATE BETWEEN 1-JAN-1990 AND NOW ; ENAME SCOTT ADAMS HIREDATE 27-JUN JUL-90

14 The GROUP BY Clause 13 Grouping performs functions on subsets of rows. Example - Sum the salaries of each department: GROUP BY DEPTNO, SUM(SAL) DEPTNO; DEPTNO SUM Example - Average salary for each job: GROUP BY JOB, AVG(SAL) JOB; JOB AVG ANALYST 3000 CLERK 1037 MANAGER 2758 PRESIDENT 5000 SALESMAN 1400

15 The HAVING Clause 14 Used to select groups of rows. Only groups which satisfy the condition of the HAVING clause appear in the result. Example - What is the average salary of departments where nobody earns less than 1000? DEPTNO, AVG(SAL) GROUP BY DEPTNO HAVING MIN(SAL) >= 1000; DEPTNO AVG We cannot simply use, as aggregates are cannot be used in this part of a query.

16 Basic SQL Summary 15 The general form of the SQL statement is shown below: GROUP BY HAVING ORDER BY column-names table-names search-condition column-name group-select-condition column-names The actual data-types and functions available vary from one DBMS product to another. The ones presented here are all from the SQL 92 standard.

CS2 Current Technologies Lecture 3: SQL - Joins and Subqueries

T E H U N I V E R S I T Y O H F R G E D I N B U CS2 Current Technologies Lecture 3: SQL - Joins and Subqueries Chris Walton (cdw@dcs.ed.ac.uk) 11 February 2002 Multiple Tables 1 Redundancy requires excess