Structured Query Language Review

ISYS 650

Language Overview

• Three major components:– Data definition language, DDL

• Create, Drop and Alter Tables or Views– Data manipulation language, DML

• Updating database:– Insert, Delete, Update

• Query database:– Select

– Data control language (DCL)• Help DBA control the database:

– Grant/revoke privileges to access the database, creating procedures, etc.

DDL

• Table structure:– Field name– Field data type– Primary key– Integrity constraints:

• Null• Domain constraint• Referential integrity• Etc.

CREATE TABLE

• CREATE TABLE tableName(fields and data type separated by commas);

• Ex.– CREATE TABLE employee(

eid CHAR(5),

ename VARCHAR(40),

sex CHAR,

salary NUMERIC(9,2),

hire_Date DATE);

Integrity Enhancement Feature• Required data: NOT NULL

– eid CHAR(5) NOT NULL,• Default value:DEFAULT

sex CHAR DEFAULT ‘M’,

• Field domain: CHECK(condition)– salary NUMERIC(9,2) CHECK (salary >= 100 AND salary

<=10000),– sex CHAR DEFAULT ‘M’ CHECK (sex in (‘M’,’F”)),

• PRIMARY KEY– PRIMARY KEY(sid)– PRIMARY KEY(sid, cid)

• Unique – allow null value, the PRIMARY KEY constraint does not allow null.– ESSN CHAR(9) UNIQUE

Example

– CREATE TABLE employee(eid CHAR(5) PRIMARY KEY,

ename VARCHAR(40),

sex CHAR DEFAULT ‘M’ CHECK (sex in (‘M’,’F’)),

salary NUMERIC(9,2),

hire_Date DATE);

Creating Table Through SubQuery

• CREATE TABLE tableName

• AS (Select query)

• Ex.– CREATE TABLE newEmp– AS (SELECT empid, ename,salary FROM

emp);

SQL Insert CommandINSERT INTO tableName VALUES (field values separated by commas);

INSERT INTO tableName (Column names separated by commas)VALUES (field values separated by commas);

Ex 1. Customer table with CID, CNAME, CITY, RATING.

a. INSERT INTO CUSTOMER VALUES (‘C1’, ‘SMITH’, ‘SF’, ‘A’);

b. INSERT INTO CUSTOMER (CID, CNAME,RATING) VALUES (‘C1’, ‘SMITH’, ‘A’);

Record with Date Field

• Oracle date format: – ‘dd-mmm-yyyy’

• Example:• insert into orders values('O7','c2','s1','10-oct-2007');

Inserting records from an existing table (Append)

• INSERT INTO stu2

• (select * from student);

SQL Delete Command

DELETE FROM tableName [WHERE criteria];

Ex 1. Delete a record from the Customer table.

DELETE FROM CUSTOMER WHERE CID = ‘C1’;

SQL Update Command

UPDATE tableName SET field = new value [WHERE criteria];

Ex 1.

UPDATE CUSTOMER SET RATING = ‘A’ WHERE CID=‘C1’;

Ex 2.

UPDATE CUSTOMER SET CITY = ‘SF’, RATING = ‘A’ WHERE CID=‘C1’;

Dealing with Null

• Null is a key word. We can use Null in the INSERT, UPDATE, and DELETE command.

• Use IS NULL (or IS NOT NULL) in a criteria.• Examples:

– INSERT INTO emp VALUES (‘e95’,’June’,’f’,NULL,5000);

– UPDATE emp SET salary=null where empid=‘e99’;– SELECT * FROM emp WHERE salary IS NULL;– SELECT * FROM emp WHERE salary= NULL (not

working);

Maintaining Referential Integrity

• FOREIGN KEY (field) REFREENCES parentable

DROP TABLE parent;CREATE TABLE parent (

pid char(3) not null,pname varchar(20),PRIMARY KEY (pid) );

drop table child;CREATE TABLE child (

cid char(3) not null,cname varchar(20),pid char(3),PRIMARY KEY (cid),FOREIGN KEY (pid) REFERENCES parentON DELETE SET NULL );

ON DELETE

• ON DELETE SET NULL

• ON DELETE CASCADE

Basic SQL Select Commands

Basic Relational Query Operations

• Selection

• Projection

• Natural Join

• Sorting

• Aggregation: Max, Min, Sum, Count, Avg– Total– Sub totals: Group By

• Calculated field

• Sub Queries

Examples

• University database:– Student: SID, Sname, Sex, Major, GPA, FID– Account: SID, Balance– Faculty: FID, Fname, Phone– Course: CID, Cname, Credits, FID– Registration: SID, CID

• Download the batch file OracleUniv.SQL to create the database.

Selection

• Selection operation works on a single relation and defines a relation that contains records that satisfy the criteria.

Selection Syntax• SELECT * FROM tableName WHERE criteria;• Criteria:

– =, <>, <, >, <=, >=– (), NOT, AND, OR– BETWEEN

• WHERE salary BETWEEN 1000 AND 10000;

– LIKE, NOT LIKE: %• WHERE ename LIKE ‘C%’

– Cannot say: WHERE ename = ‘C%’

– IN, NOT IN• WHERE eid IN (‘e1’,’e3’)

– IS NULL• WHERE rating IS NULL

– Arithmetic expression in criteria:• WHERE salary*0.1 < 500

ROWNUM Field

• ROWNUM field is a pseudocolumn that applies to every table even though it is not displayed through a SELECT * command.

• SELECT ROWNUM, empid FROM emp;

• SELECT * FROM emp WHERE ROWNUM < = 3;

Projection

• Projection operation works on a single relation and defines a vertical subset of the relation, extracting the values of specified attributes.

Projection Syntax

SELECT fields FROM tableName;– Ex: SELECT empID, ename FROM

Employee;

• Alias:• Ex: SELECT eid AS empID, ename AS empName

FROM emp;

• Projection and Selection:– SELECT fields FROM tableName WHERE

criteria;

Duplications due to Projection• Registration file:

• SID CID• ------ --------• s1 bics263• s1 acct101• s2 bics363• s2 bics464• s2 fin350• s3 bics263

• SELECT sid FROM Registration;• Note: What is the meaning of this result?

DISTINCT/UNIQUE

• SELECT DISTINCT fields FROM tableName WHERE criteria;– Ex. SELECT DISTINCT sid FROM Registration;

• UNIQUE– SELECT UNIQUET fields FROM tableName WHERE

criteria;

– Ex. SELECT UNIQUE sid FROM Registration;

Calculated Fields

• SELECT empid,ename,salary*0.1 AS tax FROM emp;

Oracle Functionshttp://download.oracle.com/docs/cd/B19306_01/server.102/b14200/toc.htm

• Text functions:– UPPER, LOWER, INITCAP

• Ex. SELECT INITCAP(Fname) FROM Faculty;– SUBSTR(text,starting position, # of chars)– LPAD(text,length of text,padding symbol)

• LPAD(salary,10,’*”)– CONCAT(text1,text2)

• We can also use concatenation operator, ||

• Date functions:– SYSDATE,NEXT_DAY

• Ex. NEXT_DAY(SYSDATE,’MONDAY’)– MONTHS_BETWEEN(date1,date2)

• Ex. SELECT Ename, MONTHS_BETWEEN(SYSDATE, Hire_Date) FROM Employee;

– ADD_MONTHS(date,# of months added)

Using the SQL CASE Function

SELECT sid, sname, CASE WHEN gpa < 2.0 THEN 'Poor'WHEN gpa < 3.0 THEN 'Good'ELSE 'Excellent'END AS GPAGroupFROM student;

Natural Join

• The two relations must have common attributes.

• Combines two relations to form a new relation where records of the two relations are combined if the common attributes have the same value. One occurrence of each common attribute is eliminated.

Natural Join Syntax• SELECT * FROM table1 NATURAL JOIN table2;

• Example:– SELECT * FROM student NATURAL JOIN faculty;

• Note: What if the FID in the student table is named AdvisorID?

Standard Join Syntax

• SELECT * FROM table1, table2 WHERE table1.JoinAttribute = table2.JoinAttribute;– SELECT * FROM student, faculty – WHERE student.fid = faculty.fid;

• Table name alias:– SELECT * FROM student s, faculty f

• WHERE s.fid = f.fid;

• On:– SELECT * FROM student s JOIN faculty f ON s.fid=f.fid;

• Difference between natural join and standard join?

Self Join

• Recursive relationship:– Employee supervise Employee

• Ex: EmpFile: empID, empName, superID• Find employees’ supervisor name:

– SELECT e1.empid, e1.empname,e2.empname as sueprname

– FROM empfile e1, empfile e2– WHERE e1.superid=e2.empid;

Sorting

• ORDER BY fieldName [DESC]– SELECT * FROM student ORDER BY sname;– SELECT * FROM student ORDER BY sname DESC;

• More than one field:– SELECT * FROM student ORDER BY major, sname;

• Note 1: Don’t name a table “ORDER” because ORDER is a SQL keyword.

• Note 2: If there is a WHERE clause, then the ORDER BY clause should be placed after the WHERE clause.

Aggregates

• SELECT AVG(fieldName) FROM tableName;– COUNT(fieldName), COUNT(*)– COUNT(DISTINCT fieldName)– MAX(fieldName)– MIN(fieldName)– SUM(fieldName)

• More than one aggregate:• SELECT AVG(fieldName), MAX(fieldName),

MIN(fieldName) FROM tableName;• With alias:

– SELECT AVG(gpa) AS avggpa, COUNT(sid) AS scount • FROM student;

GROUP BY• GROUP By one field:

– SELECT groupingFields, function(fieldname) FROM tablename GROUP BY groupingFields;

– SELECT major, count(sid) FROM student GROUP BY major;

• GROUP BY more than one field:– SELECT sex, major, count(sid) FROM student GROUP BY sex, major;

• Compute more than one subtotals:– SELECT major, count(sid), avg(gpa) FROM student – GROUP BY major

• Note 1: All grouping fields in the SELECT clause must be included in the GROUP BY clause).

• Note 2:WHERE clause must come before the GROUP BY:– SELECT major, count(sid) FROM student WHERE GPA > 3.0 GROUP BY

major;

Compute Subtotals from a Join

• Compute the number of courses taken by each student:– SELECT sid, sname, COUNT(cid)

• FROM student NATURAL JOIN registration• GROUP BY sid, sname;

• Compute the number of students advised by each faculty advisor:– SELECT fid, fname, COUNT(sid)– FROM faculty NATURAL JOIN student– GROUP BY fid, fname;

Oracle’s Natural Join May Produce Incorrect Results

select sid,sname,sum(units)from student natural join registration natural join coursegroup by sid,sname;

select sid,sname,sum(units)from (student natural join (registration natural join course))group by sid,sname;

Adding a Criteria for the Sub Totals with HAVING

• Find majors that have at least 5 students:– SELECT major, count(sid) FROM student– GROUP BY major– HAVING count(sid) > 5;

• Sometime the aggregates are not required to display:– SELECT major FROM student– GROUP BY major– HAVING count(sid) > 5;

Nesting Aggregates

• SELECT MAX(AVG(gpa))

• FROM student

• GROUP BY major;

University Database Questions• Q1: Display College of Business students’ ID and name.• Q2: Display students’ ID and name who owe university

more than $2000.• Q3: Display faculty’s name and phone if the student’s

GPA is lower than 2.0.• Q4: Display faculty’s name and phone if the faculty

advises at least 2 student.• Q5: Display students’ ID and name who are taking at

least 2 course.• Q6: Display students’ ID and name who are taking 464

and GPA < 2.0.• Q7: Display CID, Cname and the number of students in

each course.• Q8: Display SID, Sname, TotalUnits

SQL Sub (or Nested ) Query

Examples• Q: Find students whose GPA is below the

average.– The criteria itself requires a SQL statement.– SELECT * FROM student

• WHERE gpa < (SELECT AVG(gpa) FROM student);

• Q: Find employees with higher than average salary.

Sub Query with IN•SELECT sid, sname FROM student

•WHERE sid in (‘S1’, ‘S2’);•Use a subquery to retrieve keys that meet criteria.

Example: Display students’ ID, name who owe university more than $2000. (Note: not showing balance)

SELECT sid,sname FROM student WHERE SID IN (SELECT SID FROM account WHERE balance > 2000);

Example: Display students’ ID, name and Balance who owe university more than $2000.

Sub Query with IN, NOT IN• Use a subquery to retrieve keys that meet

criteria.• Q: Display faculty’s ID and name if the faculty

advises at least one student.– SELECT fid, fname FROM faculty– WHERE fid IN (SELECT DISTINCT fid FROM

student);

• Q: Display faculty’s name and name if the faculty does not advise any student.– SELECT fid, fname FROM faculty– WHERE fid NOT IN (SELECT DISTINCT fid FROM

student);

Subquery in the SELECT List

• SELECT eid,ename,salary,(select AVG(salary) from employee) as AvgSal FROM employee;

• SELECT eid,ename,salary-(select AVG(salary) from employee) as DeviateFromMean FROM employee;

Multiple Levels Subquery• Find students who are taking one-unit courses

and display their ID and name. • Join:

– Select sid, sname, cid from student natural join registration natural join course

where units=1;

• Without Join:– Select sid, sname From Student– Where sid in (select sid from registration where cid in – (select cid from course where units=1));

Correlated SubQueries

• A correlated subquery references values of the main query.

• In a correlated subquery, the main query provides values which are used by subquery’s WHERE clause.

• The subquery is executed repeatedly, once for each row that might be selected by the main query.

Passing Values from Main Query to SubQuery

• Find students taking more than 2 courses:

• select * from student s

• where (select count(cid) from registration where sid=s.sid)>2 ;

EXISTS, NOT EXISTS

• Test whether a subquery returns any rows.

• It returns True/False

Find faculty who advise at least one student.

• 1. Subquery:– SELECT fid,fname FROM faculty – WHERE fid in (SELECT DISTINCT fid FROM

student);

• 2. Correlated query:– SELECT fid,fname FROM faculty – WHERE EXISTS (SELECT * FROM student

– WHERE fid = faculty.fid);

SELECT List Containing Nested Query

SELECT sid, sname, (SELECT COUNT(cid) FROM registration WHERE sid=student.sid) AS courses

FROM student;