1 COMP 1100 Basic SQL David J. Stucki

Outline SQL Overview Retrievals Schema creation Table creation Constraints Inserts Updates 2

SQL Structured Query Language  High-level, declarative programming language User specifies what he wants, not how to do it  Comprehensive database language Statements for data definition, queries and updates Defining views, specifying authorization Setting up integrity constraints and transaction controls 3

SQL Terminology Table  Equivalent to relational model’s relation Row  Equivalent to tuple Column  Equivalent to attribute CREATE statement  Used for data definition 4

Schemas Identified by a schema name Can be thought of as a grouping of elements into one database idea  Tables  Constraints  Views  Domains  Authorizations (Roles) 5

Sample Schema 6

SQL Retrieval SQL retrieval operation: SELECT statement  SELECT FROM WHERE ;  - Projection attributes  - Selection condition 7

SQL Retrieval  Example SQL Query SELECTBIRTH_DATE, ADDRESS FROMEMPLOYEE WHEREFIRST_NAME=‘John’ AND LAST_NAME=‘Smith’;  Retrieves every row from the table where the selection condition holds true Anyone with the name “John Smith” will have their birth date and address returned 8

SQL Retrieval  Example select-project-join SQL Query SELECTFname, Lname FROMEMPLOYEE, DEPARTMENT WHEREDname=‘Research’ AND Dnumber=Dno;  Here will retrieve first name and last name of all employees who work in the ‘Research’ department Join condition – combines multiple tables in the WHERE Finds the row in the table DEPARTMENT where DNAME is ‘Reseach’ Matches the DNUMBER for that row to the DNO in the EMPLOYEE table  Only retrieves results where these values match 9

SQL Retrieval  Another example select-project-join SQL Query SELECTPname, Lname FROMEMPLOYEE, DEPARTMENT, PROJECT WHEREDnum=Dnumber AND Mgr_ssn=Ssn AND Plocation = ‘Houston’; Here we find:  Names of projects located in Houston  Last names of the managers of the departments that these projects are associated with Note that this query joins across 3 tables:  Need PROJECT table to find the location  Relate PROJECT to DEPARTMENT via DNUMBER to get Department Manager’s SSN  Use Dpt. Manager’s SSN to find last name in EMPLOYEE table 10

SQL Retrieval  Suppose we had chosen different names for our attributes What if we wanted to use “Dnumber” everywhere for the department number? This would no longer work: SELECTFname, Lname FROMEMPLOYEE, DEPARTMENT WHEREDname=‘Research’ AND Dnumber=Dnumber;  In this case, what do we mean by Dnumber=Dnumber? Ambiguous - we have two tables with the same attribute in them 11

SQL Retrieval  We can specify exactly what we mean SELECTFname, Lname FROMEMPLOYEE, DEPARTMENT WHEREDname=‘Research’ AND EMPLOYEE.Dnumber =DEPARTMENT.Dnumber;  Can also use aliases to make our lives simpler: SELECTFname, Lname FROMEMPLOYEE AS E, DEPARTMENT AS D WHEREDname=‘Research’ AND E.Dnumber=D.Dnumber; 12

SQL Retrieval  Aliases can also help us with circular references: SELECTE.Fname, E.Lname S.Fname, S.Lname FROMEMPLOYEE AS E, EMPLOYEE AS S WHEREE.Super_ssn = S.Ssn; This gets us back a list of employees and their supervisors  Without aliasing, this query would not work  Best practice is to always use aliases, even when you don’t need them  Usually improves understandability of code:  SELECTFname, Lname FROMEMPLOYEE AS EMP, DEPARTMENT AS DEPT WHEREDEPT.Dname=‘Research’ AND DEPT.Dnumber=EMP.Dno; 13

SQL Retrieval Missing “WHERE” clauses  Where clauses are not required  When missing, ALL results are returned: SELECTFname, Lname FROMEMPLOYEE; This would return all employee names in the table 14

SQL Retrieval  Need to be careful with results  When multiple tables involved, you get back the cross product of your tuples: SELECTFname, Lname, Dname FROMEMPLOYEE, DEPARTMENT This actually returns ALL employees crossed with ALL Department names  So if you have two employees and two deparments, ‘John Smith’ in Research and ‘Bob Jones’ in ‘Accounting’, you would get: John Smith Accounting John Smith Research Bob Jones Accounting Bob Jones Research 15

SQL Retrieval Wildcards in the select clause  Retrieve all of the attributes SELECT* FROMEMPLOYEE; This would return all employee data from the EMPLOYEE table 16

SQL Retrieval Duplicate elimination  In a pure relational model, no relation would have duplicate tuples  But SQL does allow duplicate tuples in a relation So long as there is no conflict in their primary keys …but if there’s no primary key defined in a table, it can have duplicate entries 17

SQL Retrieval  Even on tables that have a primary key, sometimes queries will return duplicates If we’re only getting a limited number of attributes, there could be overlap in those attributes If we want to remove duplicates, we use the DISTINCT keyword SELECTDISTINCT Salary FROMEMPLOYEE; This would return only the unique SALARY entries from the EMPLOYEE table, removing duplicate entries 18

SQL Retrieval Wildcards in the WHERE clause  Sometimes we don’t want to match a whole attribute completely A partial match is enough We can use a wildcard in the WHERE clause to accomplish this SELECTFname, Lname FROMEMPLOYEE WHEREAddress LIKE ‘%Houston%’; If we really need a ‘%’ literal character, we need to escape it: SELECTProduct_name FROMPRODUCTS WHEREDiscount LIKE ‘%5\%’; 19

SQL Retrieval Arithmetic in queries SELECTLname, 1.1*Salary AS Increase FROMEMPLOYEE; This would give us back all of the employee last names with their salary increased by 10%  Note that it doesn’t change what’s in the database – just what is reported by the query Ranges SELECT* FROMEMPLOYEE WHERESalary >= AND Salary <= ; 20

SQL Retrieval Ordering  We can force the query to come back in a particular order Very useful for reports and for debugging SELECT* FROMEMPLOYEE ORDER BYLname, Fname, Super_ssn; 21