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Structured Query Language
 

Structured Query Language

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    Structured Query Language Structured Query Language Document Transcript

    • CHAPTER : STRUCTURED QUERY LANGUAGEChapter Objectives At the end of this chapter, you should be able to:  understand the general concepts of the relational model;  explain the common integrity rules.Essential Reading An introduction to Database System Volume I (5th Edition), C.J.Date (1990), Addison Wesley. [Chapter 11 - 12] Modem Database Management (4th Edition), Fred McFadden & Jeffrey A. Hoffer (1994), Benjamin/Cummings. [Chapter 8, page 283-316]Useful Websites to learn Database and Programming:http://erwinglobio.wix.com/ittraininghttp://ittrainingsolutions.webs.com/http://erwinglobio.sulit.com.ph/http://erwinglobio.multiply.com/Prof. Erwin M. Globio, MSIT 7-1
    • DB212 CHAPTER 7: STRUCTURED QUERY LANGUAGE7-2 Prof. Erwin M. Globio, MSIT
    • DB212 CHAPTER 7: STRUCTURED QUERY LANGUAGE7.1 Relational Data Structures and Concepts Two-dimensional tables whose attributes values are atomic. This is to say that at every row-and-column position within the table, there always exists precisely one value, never a list of values. (Recalls: Relations do not contain repeating groups after INF.) There are no duplicate rows. (Recall: There is always a primary key in a relation.) Rows (tuples) are unordered. S# SNAME STATUS CITY S1 Smith 21 London S2 Jones 10 Paris S3 Mary 30 Paris S4 Clark 20 Athens <----------------------------------- Degree -------------------------------------> Degree of a relation is the number of attributes in a relation. Cardinality of a relation is the number of tuples in a relation. Domain is a pool of values, from which one of more attributes draw their actual values. For example, a domain of supplier numbers is the set of all possible supplier numbers and the domain.7.2 Integrity Rules  The entity integrity rules No component of the primary key of a relation is allowed to accept nulls. Bu "null", we mean that information is missing for some reason. Suppose in the supplier relation, there is a tuple for which the supplier-no is null. This would be like saying that there was a supplier in the real world that had no identity. This is absurd.  The referential integrity rule The database must not contain any unmatched foreign key values. By the term "unmatched foreign key value", we mean a non-null foreign key value for which there does not exist a matching value of the primary key in the relevant target relation.  The referential integrity rule says that if B references A, then A must exist. Note: Foreign key and referential integrity concepts are defined in terms of one another. Thus, "support for referential integrity" and "support for foreign keys" mean exactly the same thing.) The basic idea is as follows: For each foreign key, there are three questions that need to be answered:  Can that foreign key accept nulls? For example, does it make sense for a shipment to exist for which the supplier is unknown?Prof. Erwin M. Globio, MSIT 7-3
    • DB212 CHAPTER 7: STRUCTURED QUERY LANGUAGE  What should happen on an attempt to delete the target of a foreign key references? For example, an attempt to delete a supplier for which there exists at least at one matching shipment? Let us consider more definitely the following cases: Restricted The delete operation is "restricted" to the case where there are no such matching shipments (it is rejected otherwise) Cascades The delete operation "cascades" to delete those matching shipments also Nullifies The foreign key is set to null in all such matching shipments and the supplier is then deleted (of course, this case could not apply if the foreign key cannot accept nulls in the first place)  What should happen on an attempt to update the primary key of a foreign key reference? For example, an attempt to update the supplier number for a supplier for which there exists at least one matching shipment? Let us consider the following cases: Restricted The update operation is "restricted" to the case where there are no such matching shipments (it is rejected otherwise) Cascades The update operation "cascades" to updates the foreign key in those matching shipments also Nullifies The foreign key is set to null in all such matching shipments and the supplier is then updated (of course, this case could not apply if the foreign cannot accept nulls in the first place) Note: Restricted-Cascaded-Nullifies options for the foreign key delete and update rules do not exhaust the possibilities. They are just common practices.7.3 SQL and Relational Model SQL (Structured Query Language) is a relational language, which provides an interface between the user and the RDBMS 7.3.1 A Brief Decomposition SQLs capabilities are divided between the data description language (DDL) and the data manipulation language (DML). DDL Create tables Query table data Modify tables (ALTER) Modify table data Create indexes - Insert Drop tables - Delete Drop indexes - Update Authorization (GRANT) Aggregate functions Deauthorisation (REVOKE)7-4 Prof. Erwin M. Globio, MSIT
    • DB212 CHAPTER 7: STRUCTURED QUERY LANGUAGE The purpose of the DDL is to create entries in the catalogue (sometimes called the data dictionary or, in some large systems, the repository). The catalogue is managed by the RDBMS and it is a requirement of the Relational Model that the catalogue itself takes the form of normalized tables. The update of these tables is quite transparent to the user.7.4 Relational Data Definition Insql (DDL) Data Types DECIMAL (m,n) Signed numbers, where m is the total number of digits to the right of the decimal point. INTEGER Large (up to 11 digits) positive or negative whole numbers SMALL Small (5 or 6 digits, depending on the DBMS) positive or negative whole numbers. By specifying this data type, less storage space is required FLOAT (m,n) Whole and fractional numbers represented in scientific notation, where m is the total number of digits (including sign), and n is the number of digits to the right of the decimal point. CHAR (n) Alphanumeric (character) data, where n is the maximum length for the character string; n character positions are allocated to each instance of a CHAR column. DATE Calendar dates. A system variable can be used to set the format for the date data type. LOGICAL True or false values DDL Commands Description CREATE TABLE Define the structure of a new table INSERT Places a new row in a table based on values supplied in the statement, copies one or more rows derived from other database into a table, extracts data from one table and inserts them into another. UPDATE Changes values in one or more specified rows of a table by replacing current values with constant or the results of calculation. DELETE Removes one of more rows from a table. CREATE VIEW Names the view and optionally specifies column names to be used in place of those of the base table. CREATE INDEX Defines an index on one column that enables rapid access to the rows of a table in a sequence or randomly by the key value. ALTER TABLE Adds columns to an existing. DROP TABLE Deletes table. DROP INDEXES Deletes index. GRANT Granting user access. REVOKE Removing privileges given to a user. (DBA privileges)Prof. Erwin M. Globio, MSIT 7-5
    • DB212 CHAPTER 7: STRUCTURED QUERY LANGUAGE Illustrations: The following is the suppliers-parts-projects database SUPPLIER PROJECTS S# SNAME STATUS CITY J# JNAME CITY S1 SMITH 20 LONDON J1 SORTER PARIS S2 JONES 10 PARIS J2 PUNCH ROME S3 BLAKE 30 PARIS J3 READER ATHENS S4 CLARK 20 LONDON J4 CONSOLE ATHENS S5 ADAMS 30 ATHENS J5 COLLATOR LONDON J6 TERMINAL OSLO J7 TAPE LONDON PART P# PNAME COLOR WEIGHT CITY P1 NUT RED 12 LONDON P2 BOLT GREEN 17 PARIS P3 SCREW BLUE 17 ROME P4 SCREW RED 14 LONDON P5 CAM BLUE 14 PARIS P6 COQ RED 19 LONDON S# P# J# QTY S# P# QTY S1 P1 J1 200 S1 P1 300 S1 P1 J4 700 S1 P2 200 S2 P3 J1 400 S1 P3 400 S2 P3 J2 200 S1 P4 200 S2 P3 J3 200 S1 P5 100 S2 P3 J4 500 S1 P6 100 S2 P3 J5 600 S2 P1 300 S2 P3 J6 400 S2 P2 400 S2 P3 J7 800 S3 P2 200 S2 P5 J2 100 S4 P2 200 S3 P3 J1 200 S4 P4 300 S3 P4 J2 500 S4 P5 400 S4 P6 J3 300 S5 P6 J7 300 S5 P6 J2 200 S5 P1 J4 100 S5 P3 J4 200 S5 P4 J4 800 S5 P5 J4 400 S5 P6 J4 5007-6 Prof. Erwin M. Globio, MSIT
    • DB212 CHAPTER 7: STRUCTURED QUERY LANGUAGE Example:  Adding data to a table  Adding of one new row to a table INSERT INTO SUPPLIE VALUES ( "S6", "MARY" , 30, "NEW YORK" ) ;  Transferring rows from external file to a database table INSERT INTO SUPPLIER SELECT * FROM NEW-SUPPLIER WHERE STATUS >20 ;  Deleting database contents Rows can be deleted individually or in groups.  Deleting a row from a table DELETE FROM PART WHERE WEIGHT = 12 ;  Deleting all rows of a table DELETE FROM PART ; Note: Deletion must be done with care when rows from several relations are involved. For example, if we deleted a SUPPLIER row or a PROJECT row, or a PART row, we will have a referential integrity violation in the SPJ or SP tables.)  Changing database contents To update data in SQL, we must inform the DBMS what relation, columns and rows are involved. UPDATE SUPPLIER SET CITY = ROME WHERE SUPPLIER-NO = "S3" ;  Constructing a View for more than one table A view is also called a virtual table because it does not actually contain stored data, combines data that is selected from one or more base tables of the other views. If the data in the base tables changes, so does the information displayed using the view. We can use a view to let someone access just the rows and columns that the person needs from base tables. We can omit columns containing irrelevant or sensitive information. For instance, CREATE VIEW SUPPLIER-PART (SUPPLIER-NO, PART-NO,QUANTITY) AS SELECT S#, P#, QTY FROM SP WHERE QTY > = 200;Prof. Erwin M. Globio, MSIT 7-7
    • DB212 CHAPTER 7: STRUCTURED QUERY LANGUAGE This will create an external presentation to the user on the table SP. CREATE VIEW DEALS (SUPPLIER-NAME, CITY, PROJRCT) AS SELECT SNAME, SUPPLIER. CITY, JNAME FROM SUPPLIER, PROJECT WHERE SUPPLIER, CITY = PROJECT. CITY; This view links suppliers with the project. The two tables are related using their common CITY column.  Creating indexes You can create indexes in most RDBMSs to provide rapid random and sequential access to base-table data. For example, to create an index on the supplier table for the S#: CREATE INDEX S-NO ON SUPPLIER (S#);  Changing table definitions We may change table definitions in many SQL RDBMSs by ALTERING column specifications. For example, to add a new column ADDRESS in the SUPPLIER table: ALTER TABLE SUPPLIER ADD (ADDRESS CHAR (30) ) ;  Deleting tables Once a table is deleted, all the data will be deleted permanently and you cannot get it back. DROP VIEW tablename;  Deleting views Here, the TABLES(s) on which the VIEW is based will not be changed. DROP INDEX indexname;7-8 Prof. Erwin M. Globio, MSIT
    • DB212 CHAPTER 7: STRUCTURED QUERY LANGUAGE7.5 Basic SQL Construct (DML) SQLs DML is divided into two parts, retrieval commands and updates commands. The following are the eight operators used by the RM (Relational Model) for retrieval of data from normalized relations: Restrict Project a X a x b y b y c c x a y b x c y Union Intersection Difference These represent the eight original operators used by the relational model for retrieval model for retrieval of data from normalization relations. Product, union, insertion and difference are operators in a branch of mathematics called set theory. Restrict, project, join and divide are operators in a branch of mathematics called relational theory.Prof. Erwin M. Globio, MSIT 7-9
    • DB212 CHAPTER 7: STRUCTURED QUERY LANGUAGE SELECT Columns to be projected out eventually. SELECT * To display all data in a table. SELECT DISTINCT To display only one of the number of identical rows. SELCT <column name. .> To display data from specific rows. WHERE Search condition for the rows. SQL Logical Operations NOT Select rows that do not meet the search condition. AND Both conditions must be true to qualify the row. OR One or both of the conditions must be true. Predicates used: BETWEEN Select column values that fall within a range. In Selects column values that match one of a number of values. ANY Select rows whose values match any of the values returned by the inner query. ALL Select rows whose values have some relation to all of the values returned by the inner query. EXISTS The EXISTS predicate determines whether the outer WHERE condition is true or false. If the inner query returns any value, the WHERE condition is true. Otherwise, the condition is false. Thus a simple subquery that uses EXISTS returns all rows if the outer WHERE condition is true. LIKE Selects column values that contain a character string Underscore (-) - represent any single character. Percent symbol (%) - represent no characters or a number of characters. GROUP BY Rearranges the rows extracted into partitions or groups, such that within any one group, all rows have the same value for the GROUP BY field. HAVING Conditions applied to each group from previous GROUP BY clause. ORDER BY Sort required columns either in ascending or descending order. Aggregate functions: Number of values in the column COUNT Sum of the values in the column SUM Average of the values in the column AVG Largest value in the column MAX Smallest value in the column7 - 10 Prof. Erwin M. Globio, MSIT
    • DB212 CHAPTER 7: STRUCTURED QUERY LANGUAGE  Restrict Restriction retrieves rows from a relation based upon selection criteria. Thus, SELECT * (* means all columns) FROM S (target table) WHERE SCITY= `LONDON ; (selection criterion) will retrieve the following: S# SNAME STATUS CITY S1 SMITH 20 LONDON S2 CLARK 20 LONDON  Project Projection retrieves columns form a relation based upon selection criteria. For example, SELECT S#, SNAME (named columns) FROM SUPPLIER WHERE SUPPLIER.CITY = 1LONDON ; will retrieve the following: S# SNAME S1 SMITH S2 CLARK What would the following achieve? SELECT STATUS FROM SUPPLIER WHERE SUPPLIER.CITY = `LONDON : The answer is STATUS 20 20 Is this a proper relation? (Hint: it contains duplicate rows) This can be solved easily by using the command called DISTINCT, i.e. SELECT DISTINCT STATUS FROM SUPPLIER WHERE SUPPLIER.CITY = `LONDON ; which eliminates duplicates from the resultant relation.Prof. Erwin M. Globio, MSIT 7 - 11
    • DB212 CHAPTER 7: STRUCTURED QUERY LANGUAGE  Product This is an expensive operation. For example, Let S is a relation with 5 rows; P is a relation with 6 rows. Then the product S x P will contain 30 rows, i.e. [R1 (S) R1(P), R1(S) R1(P), . . . R5(S) R6(P) ] The importance of product is that it provides the basis for the relational operator join. The following will result in a product of S, P SELECT * (all columns) FROM SUPLIER, PART; (from both tables SUPPLIER & PART) Note: Such a product is called the Cartesian product and is specifically discouraged by the Relational Model. For example, if 2 tables, each has a million entries, so if combined, it would tie up computing resources. Union Union can be described as a collection of columns that exist in one relation, or both. Each column must be of the same type and size and be drawn from the same domain. For example, if we look at SUPPLIER and PART tables, only one column qualifies (CITY). SELECT # FROM SUPLIER WHERE SUPPLIER.CITY = `LONDON UNION SELECT S# FROM SP WHERE P# = `P2 ; This will extract S# for those suppliers who supply the part P2 or who are located to LONDON, or both. The answer is S# S1 S2 S3 Duplicates are automatically eliminated. It should be noted that UNION has an associative property inbuilt, i.e. A UNION B and B UNION A are identical. Similarly, (A UNION B) UNION C is equal to (A UNION C) UNION B.  Intersection Two (or more) tables which share one (or more) union-compatible attributes may be intersected. However, SQL requires special construct to achieve this, each involving what is called sub-query. For example, we went to list what suppliers have supplied (some) parts. SELCT S# FROM SUPPLIER WHERE S# IN (SELECT S# FROM SP) ; This will find all S# in SUPPLIER which match some S# in SP.7 - 12 Prof. Erwin M. Globio, MSIT
    • DB212 CHAPTER 7: STRUCTURED QUERY LANGUAGE Is this operation equivalent to equijoin operation? Note: The outer SELCT list is not restricted. Thus, SELECT * FROM SUPPLIER WHERE S# IN (SELECT S# FROM SP) ; is perfectly fine, since only S# in SUPPLIER and S# in SP are union compatible. The Existential Quantifier `EXISTS". The above example can also be written as SELECT S# FROM S WHERE EXISTS (SELECT * FROM SP WHERE S.S# = SP.S#); Note: The `SELECT * in the subquery does not have the meaning `select all columns as you have been as far led to believe: it means `scan all rows!) The method of execution is as follows: `For each S# in S, scan all rows in SP until the expression S.S# is satisfied the search is exhausted, which ever occurs first. Return `YES if a match is made, `NO otherwise. If `YES, then the current S# in S is selected. In other words, `EXISTS returns a binary valued response whereas `IN returns a relation. `EXISTS" can always be used in place of `IN, but not vice versa.  Difference Difference is the negation of two relation. For example, SELECT S# FROM SUPPLIER WHERE NOT S# IN (SELECT S# FROM SP) ; (Think about: Is negation associative?)  Divide For example, SP divide by PART will give S1 only. SP divide by P S1 P1 P1 S1 P2 P2 S1 P3 P3 S1 P4 P4 S1 P5 P5 Since this is not commonly represented in SQL.Prof. Erwin M. Globio, MSIT 7 - 13
    • DB212 CHAPTER 7: STRUCTURED QUERY LANGUAGE  Join The purpose of a join is to create a new relation from two or more existing relations. In SQL, this is achieved implicitly. This is no JOIN command in SQL. There are some rules involved:  In order for two tables to be joined, they must share an attribute whose values are taken from the same domain.  The result table must be a relation. Thus, if it has no primary key, the select list must include `DISTINCT to remove duplicate rows.  Where the result table may contain duplicate columns (SQL does not eliminate them), the SELECT * construct is therefore discouraged.  The most meaningful join is one which joins a primary key to its foreign key(s). (Recall: Normalization helps to decompose a complex model and JOIN is used to reconstruct the original model.) For example, we want to know for all suppliers, what parts and how many they have supplied. SELECT SUPPLIER.S#, SNAME, P#, QTY FROM SUPPLIER, SP (Tables to be joined) WHERE SUPPLIER.S# = SP.S#; If we want to extract the name of the part supplied, then SELECT SUPPLIER.S#, SNAME, SP.P#,PNAME,QTY FROM SUPPLIER, SP, PART WHERE SUPPLIER.S# = SP.S# AND SP.P# = PART.P# ; We can also use the greater sign `>, or smaller sign `<, or even the not equal sign `<> in the WHERE condition.7 - 14 Prof. Erwin M. Globio, MSIT
    • DB212 CHAPTER 7: STRUCTURED QUERY LANGUAGE7.6 Review Questions 1. What are the differences between equijoin and natural join. Illustrate with examples. [4 marks] 2. a. Answer the following questions using the below table structures. Student Course Student-ID Numeric 5 Course-ID Character 5 Name Character 15 Title Character 20 Major Character 3 Instructor Character 10 Registration Student-ID Numeric 5 Instructor Name Character 15 Instructor Character 10 Major Character 3 Location Character 4 i. Write a full database description using the SQL data definition language for the above table Student. [2 marks] ii. Write a SQL command to add a new student "John" with student number 12221 and majoring in "IT". [1 mark] iii. Define the following view by SQL definition. [2 marks] Student-ID Name Major Course-ID Grade iv. Write a SQL command to change the grade for student number 38214 from 51 to 57 [2 marks] v. Write a SQL command to display the average grade for each course. [3 marks] vi. Write a SQL command to count the number of students taught by each instructor. [3 marks] vii. Write a SQL command to display an ordered list of courses and the respective instructors, of which all students grades in those courses are above 50. [3 marks] b. Use SQL commands to illustrate: i. Natural join [2 marks] ii. Search condition where exact matches are necessary [2 marks]Prof. Erwin M. Globio, MSIT 7 - 15
    • DB212 CHAPTER 7: STRUCTURED QUERY LANGUAGEUseful Websites to learn Database and Programming:http://erwinglobio.wix.com/ittraininghttp://ittrainingsolutions.webs.com/http://erwinglobio.sulit.com.ph/http://erwinglobio.multiply.com/7 - 16 Prof. Erwin M. Globio, MSIT