Introduction to Databases Chapter 7: Data Access and Manipulation

SELECT Statement Primary statement for data retrieval Clauses let you specify precise information retrieved, sorted, organized, and grouped Source can be tables, views, and table- valued user-defined functions Most basic syntax is: SELECT column_list FROM source WITH search_conditions

SELECT Statement Advanced Syntax SELECT column_list [ALL | DISTINCT] [TOP expression [PERCENT] [WITH TIES]] [INTO table_name] FROM source [[INNER|OUTER|CROSS] JOIN] WHERE search_condition [ORDER BY order_expression [ASC] | [DESC]] [GROUP BY [ALL] group_expression] [HAVING search_condition] Keywords: ALL DISTINCTTOP INTO JOIN ORDER BY GROUP BY HAVING

Filtering Results Filter results using comparison operators: =, >, >=,, != AND and OR: To combine conditions SELECT CUSTNUM, CUSTNAME FROM CUSTOMER WHERE HQCITY=‘New York’ OR (CUSTNUM>1500 AND HQCITY=‘Atlanta’) BETWEEN and IN: For ranges SELECT CUSTNUM, CUSTNAME FROM CUSTOMER WHERE CUSTNUM BETWEEN 1000 AND 1700 LIKE: Adds flexibility SELECT CUSTNAME FROM CUSTOMER WHERE CUSTNAME LIKE ‘C_t%’

Managing Result Sets Specifying columns SELECT PRODNUM, (QOH * UNITPRICE) AS [VALUE] FROM PRODUCT Specifying rows SELECT TOP 3 PRODNUM, (QOH * UNITPRICE) AS [VALUE] FROM PRODUCT Writing results to new table SELECT PRODNUM, (QOH * UNITPRICE) AS [VALUE] INTO PRODVAL FROM PRODUCT Limiting results to unique values SELECT DISTINCT SPNUM FROM CUSTOMER –With DISTINCT, entire result must be duplicated for row to be dropped

Sorting and Organizing ORDER BY: Defines order; can use multiple columns SELECT * FROM SALES ORDER BY SPNUM, PRODNUM ASC and DESC: Returns rows in ascending or descending order SELECT PRODNUM, PRODNAME, UNITPRICE FROM PRODUCT ORDER BY UNITPRICE DESC

Grouping GROUP BY: To group rows in result in order to perform functions such as SUM SELECT SPNUM, SUM(QUANTITY) AS [SUM] FROM SALES WHERE SPNUM> =150 GROUP BY SPNUM GROUP BY column list and ORDER BY column list must be aggregate values or called out in GROUP BY clause GROUP BY can use HAVING clause to limit results from group calculations performed by built- in function SELECT SPNUM, SUM(QUANTITY) AS [SUM] FROM SALES WHERE SPNUM>=150 GROUP BY SPNUM HAVING SUM(QUANTITY)>=5000

Operator Precedence SQL Server Transact SQL Precedence Level 1: ~ Level 2: *, /, % Level 3: +, -, & Level 4: =, >, =,, !>, !<, Level 5: ^ Level 6: NOT Level 7: AND Level 8: ALL, ANY, BETWEEN, IN, LIKE, SOME Level 9: =

Operator Precedence Operators at same precedence level evaluated from left to right When same symbol used for two operations (e.g. “ =”), type of operation being performed can be determined by context in which symbol used Precedence varies by DBMS implementation

Combining Statement Results To combine, statements must have: –Same number of columns –Columns in same order –Columns with compatible data types UNION: Combines two results SELECT HQCITY FROM CUSTOMER WHERE SPNUM = 137 UNION SELECT HQCITY FROM CUSTOMER WHERE SPNUM = 186 EXCEPT: Returns results in first query that do not appear in second INTERSECT: Returns values that appear in both queries

Using SELECT with Other Commands SELECT can be value source in other statements Replacing values clause: INSERT EmpCopy SELECT SPNUM, SPNAME, SPFIRSTNAME FROM SALESPERSON Replacing scalar input values: UPDATE SALESPERSON SET COMMPERCT = (SELECT AVG(COMMPERCT) FROM SALESPERSON)

Joins and Subqueries Join: –Combines columns from two sources –Can combine with other joins to join more than two sources Subquery: –Uses multiple SELECT statements in single query –Nested query; can include several layers of nestings, with innermost executed first

Joins Two specifications must be made in SELECT statement: 1.Tables to join must be listed in FROM clause 2.Join attributes in tables must be declared and matched If same column name used in both tables, qualify column names with table name in SELECT statement SELECT SPNAME FROM SALESPERSON, CUSTOMER WHERE SALESPERSON.SPNUM=CUSTOMER.SPNUM AND CUSTNUM=1525 Declaring and using table alias: SELECT SPNAME FROM SALESPERSON s, CUSTOMER c WHERE s.SPNUM=c.SPNUM AND CUSTNUM=1525

Joins Joining three tables SELECT PRODNAME FROM SALESPERSON, PRODUCT, SALES WHERE SALESPERSON.SPNUM=SALES.SPNUM AND SALES.PRODNUM=PRODUCT.PRODNUM AND SPNAME=’Adams’ AND QUANTITY>2000 Types of Joins: –Inner join: Only qualifying results are returned –Outer join: Returns qualifying rows from one table and all rows from other (outer) table –Cross join: Returns all possible rows, whether or not they meet qualifying join logic, in every possible combination

Different Join Syntaxes Inner join SELECT SPNAME, CUSTNAME FROM SALESPERSON JOIN CUSTOMER ON (SALESPERSON.SPNUM=CUSTOMER.SPNUM) Outer join –May be LEFT or RIGHT, returning unqualified results from either table to left or right of join syntax SELECT SPNAME, CUSTNAME FROM SALESPERSON LEFT OUTER JOIN CUSTOMER ON (SALESPERSON.SPNUM=CUSTOMER.SPNUM) Cross join SELECT SPNAME, CUSTNAME FROM SALESPERSON CROSS JOIN CUSTOMER

Basic Subqueries Subqueries: –Nested queries using several levels of SELECT statements contained in pairs of parentheses Noncorrelated subquery: –Simplest type of subquery –Inner query does not depend on outer query –Nested (inner) query is used to retrieve a value that is passed up to outer query and used as input to SELECT statement

Basic Subqueries Noncorrelated subquery (cont.): –Used in qualifying logic in WHERE clause –Innermost SELECT statement executed first; results are provided as input to SELECT statement at next level up –Can be used as alternative to join SELECT SPNAME FROM SALESPERSON WHERE SPNUM=(SELECT SPNUM FROM CUSTOMER WHERE CUSTNUM=1525)

Batches and Scripts Batch: Set of SQL commands that run as group SELECT * INTO ECOPY FROM EMPLOYEES SELECT * FROM ECOPY Script: –Automates periodic procedures –May contain batches USE GeneralHardware GO CREATE TABLE EmpTest (SPNUM CHAR(3), SPNAME VARCHAR(20)) GO INSERT EmpTest SELECT SPNUM, SPNAME FROM SALESPERSON

Programming Concepts: Variables Variables: –Used to temporarily store value in memory –Include Identifier (unique name) – Local variable names begin symbol Data type To create variable: DECLARE To assign value: SET, SELECT int = 5

Programming Concepts: Variables Value may be static (constant) or expression Value must be compatible with data type REAL = (SELECT AVG(COMMPERCT) FROM SALESPERSON) UPDATE SALESPERSON SET COMMPERCT

Programming Concepts: Control Statements Allow batch to: –Make decisions based on values –Control order in which statements are executed –Repeatedly loop through statements Common controls: –BEGIN and END –IF…ELSE

Programming Concepts: Control Statements IF boolean_expression [BEGIN] statement_block [END] [ELSE [BEGIN] statement_block [END]]

Summary SELECT statement basic syntax: SELECT column_list FROM source WITH search_conditions. Advanced SELECT syntax keywords: ALL, DISTINCT, TOP, INTO, JOIN, ORDER BY, GROUP BY, HAVING. SELECT can be used as value source for other statements. Comparison operators used to filter and modify results: AND, OR, BETWEEN, IN, and LIKE. Result sets can be limited to specified columns or rows, written to new tables, limited to unique values, ordered, organized, and grouped.

Summary Operator precedence: Defines execution order of operators. Joins: Combine columns from two or more tables. Subqueries: Nested queries using several levels of SELECT statements contained in pairs of parentheses. Batches: Used to execute two or more statements as group. Scripts: Used to automate procedures; may be constructed using variables and control statements.

Key Terms AND Bitwise operations Boolean expression Cartesian product Control statement Control-of-flow statement Correlated subquery Cross join Identifier Inner join Join Local variable Noncorrelated subquery OR Outer join Subquery Table alias Table order User-defined function Variable XML document fragment

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