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Fundamentals of Database Systems Chapter 8 SQL99 - The Relational Database Standard IM ISU Database 1 Data Definition in SQL Schema and Catalog Concepts in SQL2 An SQL schema is identified by » A schema name » An authorization identifier to indicate the user or account who owns the schema » Descriptors for each element in the schema Schema elements include the tables, constraints, views, domains, and other constructs (such as authorization grants) that describe the schema IM ISU Database 2 Data Definition in SQL (cont.) A schema is created via the CREATE SCHEMA statement » Example Create a schema called COMPANY, owned by the user with authorization identifier JSMITH: CREATE SCHEMA COMPANY AUTHORIZATION JSMITH; IM ISU Database 3 Data Definition in SQL (cont.) Catalog » A named collection of schemas in an SQL environment » A catalog always contains a special schema called INFORMATION_SCHEMA, which provides information on all the element descriptors of all the schemas in the catalog to authorized users IM ISU Database 4 Data Definition in SQL (cont.) Purpose Used to CREATE, DROP, and ALTER the descriptions of the tables (relations) of a database CREATE TABLE Specifies a new base relation by giving it a name, and specifying each of its attributes and their data types IM ISU Database 5 Data Definition in SQL (cont.) Data types in SQL2 » Numeric – INT or INTEGER – SMALLINT – FLOAT – REAL – DOUBLE PRECISION – DEC(i, j) or DECIMAL(i,j) or NUMERIC(i,j) » Character-string – CHAR(n) or CHARACTER(n) – VARCHAR(n) or CHAR VARYING(n) or CHARACTER VARYING(n) IM ISU Database 6 Data Definition in SQL (cont.) » Bit-string – BIT(n) – BIT VARYING(n) » Date – In the form YYYY-MM-DD » Time – In the form HH:MM:SS » TIME(i) – Made up of hour:minute:second plus i additional digits specifying fractions of a second – format is hh:mm:ss:ii...i IM ISU Database 7 Data Definition in SQL (cont.) » TIMESTAMP – Has both DATE and TIME components » INTERVAL – Specifies a relative value rather than an absolute value – Can be DAY/TIME or YEAR/MONTH intervals – Can be positive or negative – when added to or subtracted from an absolute value, the result is an absolute value IM ISU Database 8 Data Definition in SQL (cont.) In SQL2, it is possible to specify the domain of each attribute and declare the domain name, e.g., CREATE DOMAIN SSN_TYPE AS CHAR(9); » We can use SSN_TYPE in place of CHAR(9) IM ISU Database 9 Data Definition in SQL (cont.) A constraint NOT NULL may be specified on an attribute CREATE TABLE DEPARTMENT ( DNAME VARCHAR(10) NOT NULL, DNUMBER INTEGER NOT NULL, MGRSSN CHAR(9), MGRSTARTDATE CHAR(9) ); In SQL2, the CREATE TABLE can be used to specify the primary key, secondary keys, and referential integrity constraints (foreign keys) IM ISU Database 10 Data Definition in SQL (cont.) » Primary key attributes can be specified via the PRIMARY KEY » Key attributes can be specified via UNIQUE » Referential integrity is specified via the FOREIGN KEY clause CREATE TABLE DEPT (DNAME VARCHAR(10) NOT NULL, DNUMBER INTEGER NOT NULL, MGRSSN CHAR(9), MGRSTARTDATE CHAR(9), PRIMARY KEY (DNUMBER), UNIQUE (DNAME), FOREIGN KEY (MGRSSN) REFERENCES EMP); IM ISU Database 11 Data Definition in SQL (cont.) It is also possible to define a default value for an attribute by appending the clause DEFAULT <value> to an attribute definition IM ISU Database 12 Data Definition in SQL (cont.) IM ISU Database 13 Data Definition in SQL (cont.) In SQL2, action can be specified if a referential integrity constraint is violated upon deletion of a referenced tuple or upon modification of a referenced primary key » Attaching a referential triggered action clause to any foreign key constraint » The options include SET NULL, CASCADE, and SET DEFAULT » An option must be qualified with either ON DELETE or ON UPDATE IM ISU Database 14 Data Definition in SQL (cont.) » SET NULL (DEFAULT) ON DELETE: the value of the deleted referencing attributes is changed to NULL or the specified default value » SET NULL (DEFAULT) ON UPDATE: the value of the updated referencing attributes is changed to NULL or the specified default value » CASCADE ON DELETE: delete all the referencing tuples » CASCADE ON UPDATE: change the value of the foreign key to the updated (new) primary key value for all referencing tuples IM ISU Database 15 Data Definition in SQL (cont.) » A constraint may be given a name, following the keyword CONSTRAINT » The names of all constraints within a particular schema must be unique IM ISU Database 16 Data Definition in SQL (cont.) Base tables (or base relations) » The relations declared through CREATE TABLE statements are called » The relation and its tuples are actually created and stored as a file by the DBMS Virtual relations » The relations created through the CREATE VIEW statement » The relations may or may not correspond to an actual physical file IM ISU Database 17 Data Definition in SQL (cont.) DROP SCHEMA Used to a whole schema There are two drop behavior options » CASCADE: remove the database schema and all its tables, domains, and other elements, e.g., DROP SCHEMA COMPANY CASCADE; » RESTRICT: the schema is dropped only if it has no elements IM ISU Database 18 Data Definition in SQL (cont.) DROP TABLE Used to remove a relation (base table) and its definition The relation can no longer be used in queries, updates, or any other commands since its description no longer exists DROP TABLE DEPENDENT; IM ISU Database 19 Data Definition in SQL (cont.) Two options » CASCADE: all constraints and views that reference the table are dropped automatically along with the table itself DROP TABLE DEPENDENT CASCADE; » RESTRICT: a table is dropped only if it is not referenced in any constraints or views IM ISU Database 20 Data Definition in SQL (cont.) ALTER TABLE Used to change the base relation definition Possible actions include » adding or dropping a column (attribute) » changing a column definition » adding or dropping table constraints Example ALTER TABLE COMPANY.EMPLOYEE ADD JOB VARCHAR(12); IM ISU Database 21 Characteristics of Relations » The new attribute will have NULLs in all the tuples of the relation » Hence, the NOT NULL constraint is not allowed for such an attribute » The database users must still enter a value for the new attribute JOB for each EMPLOYEE tuple » This can be done using the UPDATE command IM ISU Database 22 Characteristics of Relations To drop a column, we must choose either CASCADE or RESTRICT for drop behavior » CASCADE: all constraints and views that reference the column are dropped automatically along with the column » RESTRICT: the column is drop only if no views or constraints reference the column » Example ALTER TABLE COMPANY.EMPLOYEE DROP ADDRESS CASCADE; IM ISU Database 23 Characteristics of Relations It is also possible to alter a column definition by dropping an existing default clause or by defining a new default clause » Example 1 ALTER TABLE COMPANY.DEPARTMENT ALTER MGRSSN DROP DEFAULT; » Example 2 ALTER TABLE COMPANY.DEPARTMENT ALTER MGRSSN SET DEFAULT "333445555"; IM ISU Database 24 Retrieval Queries in SQL Preliminary SQL has one basic statement for retrieving information from a database; the SELECT statement This is not the same as the SELECT operation of the relational algebra IM ISU Database 25 Retrieval Queries in SQL (cont.) Important distinction between SQL and the formal relational model » SQL allows a table (relation) to have two or more identical tuples » Hence, an SQL relation (table) is a multi-set (sometimes called a bag) of tuples; it is not a set of tuples » SQL relations can be constrained to be sets by specifying PRIMARY KEY or UNIQUE attributes, or by using the DISTINCT option in a query IM ISU Database 26 Retrieval Queries in SQL (cont.) Basic form of the SQL SELECT statement SELECT <attribute list> FROM <table list> WHERE <condition> » <attribute list> is a list of attribute names whose values are to be retrieved by the query » <table list> is a list of the relation names required to process the query » <condition> is a conditional (Boolean) expression that identifies the tuples to be retrieved by the query IM ISU Database 27 Retrieval Queries in SQL (cont.) Simple SQL Queries Basic SQL queries correspond to using the SELECT, PROJECT, and JOIN operations of the relational algebra All subsequent examples use the COMPANY database Example of a simple query on one relation IM ISU Database 28 Retrieval Queries in SQL (cont.) IM ISU Database 29 Retrieval Queries in SQL (cont.) IM ISU Database 30 Retrieval Queries in SQL (cont.) Query 0: Retrieve the birthdate and address of the employee whose name is 'John B. Smith' Q0: SELECT BDATE, ADDRESS FROM EMPLOYEE WHERE FNAME='John' AND MINIT='B' AND LNAME='Smith’; » Similar to a SELECT-PROJECT pair of relational algebra operations – the SELECT-clause specifies the projection attributes – the WHERE-clause specifies the selection condition IM ISU Database 31 Retrieval Queries in SQL (cont.) BDATE,ADDRESS (FNAME=‘John’ AND MINIT=‘B’ AND LNAME=‘Smith’ (EMPLOYEE)) » However, the result of the query may contain duplicate tuples » Result of Q0 BDATE ADDRESS 1965-01-09 IM ISU 731 Fondren, Houston, TX Database 32 Retrieval Queries in SQL (cont.) Query 1: Retrieve the name and address of all employees who work for the 'Research' department Q1: SELECT FROM WHERE FNAME, LNAME, ADDRESS EMPLOYEE, DEPARTMENT DNAME='Research' AND DNUMBER=DNO; » Similar to a SELECT-PROJECT-JOIN sequence of relational algebra operations – (DNAME='Research') is a selection condition – (DNUMBER=DNO) is a join condition IM ISU Database 33 Retrieval Queries in SQL (cont.) Query 2: For every project located in 'Stafford', list the project number, the controlling department number, and the department manager's last name, address, and birthdate. Q2: SELECT FROM WHERE IM ISU PNUMBER, DNUM, LNAME, BDATE, ADDRESS PROJECT, DEPARTMENT, EMPLOYEE DNUM=DNUMBER AND MGRSSN=SSN AND PLOCATION='Stafford’; Database 34 Retrieval Queries in SQL (cont.) » In Q2, there are two join conditions » The join condition DNUM=DNUMBER relates a project to its controlling department » The join condition MGRSSN=SSN relates the controlling department to the employee who manages that department IM ISU Database 35 Retrieval Queries in SQL (cont.) Aliases (Renaming) with SQL In SQL, we can use the same name for two (or more) attributes as long as the attributes are in different relations A query that refers to two or more attributes with the same name must qualify the attribute name with the relation name by prefixing the relation name to the attribute name » e.g., EMPLOYEE.LNAME, DEPARTMENT.DNAME IM ISU Database 36 Retrieval Queries in SQL (cont.) Some queries need to refer to the same relation twice; In this case, aliases are given to the relation name Query 8: For each employee, retrieve the employee's name, and the name of his or her immediate supervisor Q8: SELECT FROM WHERE IM ISU E.FNAME, E.LNAME, S.FNAME, S.LNAME EMPLOYEE E S E.SUPERSSN=S.SSN; Database 37 Retrieval Queries in SQL (cont.) » In Q8, the alternate relation names E and S are called aliases for the EMPLOYEE relation » We can think of E and S as two different copies of the EMPLOYEE relation – E represents employees in the role of supervisees – S represents employees in the role of supervisors » We an also use the AS keyword to specify aliases, e.g., for Q8 FROM EMPLOYEE E S FROM IM ISU EMPLOYEE AS E, EMPLOYEE AS S Database 38 Retrieval Queries in SQL (cont.) » Aliasing can also be used in any SQL query for convenience Q1B: SELECT E.FNAME, E.NAME, E.ADDRESS FROM EMPLOYEE E, DEPARTMENT D WHERE D.NAME=‘Research’ AND D.DNUMBER=E.DNUMBER; IM ISU Database 39 Retrieval Queries in SQL (cont.) Unspecified WHERE-clause A missing WHERE-clause indicates no condition; hence, all tuples of the relations in the FROM-clause are selected This is equivalent to WHERE TRUE Query 9: Retrieve the SSN values for all employees Q9: IM ISU SELECT FROM SSN EMPLOYEE; Database 40 Retrieval Queries in SQL (cont.) If more than one relation is specified in the FROM-clause and there is no join condition, then the CARTESIAN PRODUCT of tuples is selected » Query 10: Retrieve all combinations of EMPLOYEE SSN and DEPARTMENT DNAME Q10: SELECT SSN, DNAME FROM EMPLOYEE, DEPARTMENT; » It is extremely important not to overlook specifying any selection and join conditions in the WHERE-clause IM ISU Database 41 Retrieval Queries in SQL (cont.) Use of * To retrieve all the attribute values of the selected tuples, a * is used, which stands for all the attributes Query 1C: Retrieves all the attribute values of EMPLOYEE at DEPARTMENT number 5 Q1C: SELECT FROM WHERE IM ISU * EMPLOYEE DNO=5; Database 42 Retrieval Queries in SQL (cont.) Query Q1D: Retrieves all the attributes of an EMPLOYEE and the attributes of the DEPARTMENT he or she works in, for every employee of the ‘Research’ department Q1D: SELECT FROM WHERE IM ISU * EMPLOYEE, DEPARTMENT DNAME='Research' AND DNO=DNUMBER; Database 43 Retrieval Queries in SQL (cont.) Use of DISTINCT SQL does not treat a relation as a set; duplicate tuples can appear To eliminate duplicate tuples in a query result, the keyword DISTINCT is used Q11: SELECT FROM Q11A: SELECT FROM IM ISU SALARY EMPLOYEE; DISTINCT SALARY EMPLOYEE; Database 44 Retrieval Queries in SQL (cont.) » The result of Q11 may have duplicate SALARY values whereas Q11A does not have any duplicate values IM ISU Database 45 Retrieval Queries in SQL (cont.) Set Operations SQL has directly incorporated some set operations There is a union operation (UNION), and in some versions of SQL there are set difference (MINUS) and intersection (INTERSECT) operations The resulting relations of these set operations are sets of tuples; duplicates are eliminated IM ISU Database 46 Retrieval Queries in SQL (cont.) The set operations apply only to union compatible relations Query 4: Make a list of all project numbers for projects that involve an employee whose last name is 'Smith' as a worker or as a manager of the department that controls the project IM ISU Database 47 Retrieval Queries in SQL (cont.) Q4: IM ISU (SELECT PNAME FROM PROJECT, DEPARTMENT, EMPLOYEE WHERE DNUM=DNUMBER AND MGRSSN=SSN AND LNAME='Smith'); UNION (SELECT PNAME FROM PROJECT, WORKS_ON, EMPLOYEE WHERE PNUMBER=PNO AND ESSN=SSN AND LNAME='Smith'); Database 48 Retrieval Queries in SQL (cont.) Substring Comparison The LIKE comparison operator is used to compare partial strings Two reserved characters are used » '%' (or '*' in some implementations) replaces an arbitrary number of characters » '_' replaces a single arbitrary character Query 12: Retrieve all employees whose address is in Houston, Texas IM ISU Database 49 Retrieval Queries in SQL (cont.) Q12: SELECT FNAME, LNAME FROM EMPLOYEE WHERE ADDRESS LIKE '%Houston,TX%’; Query 12A: Retrieve all employees who were born during the 1950s Q12A: SELECT FNAME, LNAME FROM EMPLOYEE WHERE BDATE LIKE '__5_______’; » Here, '5' must be the 3th character of the string IM ISU Database 50 Retrieval Queries in SQL (cont.) Arithmetic Operations The standard arithmetic operators '+', '-'. '*', and '/' can be applied to numeric values in an SQL query result Query 13: Show the effect of giving all employees who work on the 'ProductX' project a 10% raise Q13: SELECT FNAME, LNAME, 1.1*SALARY FROM EMPLOYEE, WORKS_ON, PROJECT WHERE SSN=ESSN AND PNO=PNUMBER AND PNAME='ProductX’; IM ISU Database 51 Retrieval Queries in SQL (cont.) ORDER BY The ORDER BY clause is used to sort the tuples in a query result based on the values of some attribute(s) The default order is in ascending order Query 15: Retrieve a list of employees and the projects each works in, ordered by the employee's department, and within each department ordered alphabetically by employee last name IM ISU Database 52 Retrieval Queries in SQL (cont.) Q15:SELECT DNAME, LNAME, FNAME, PNAME FROM DEPARTMENT, EMPLOYEE, WORKS_ON, PROJECT WHERE DNUMBER=DNO AND SSN=ESSN AND PNO=PNUMBER ORDER BY DNAME, LNAME, FNAME; We can specify the keyword DESC if we want a descending order ORDER BY DNAME DESC, LNAME ASC, FNAME ASC IM ISU Database 53 More Complex Queries Nesting of Queries A complete SELECT query, called a nested query , can be specified within the WHERE-clause of another query, called the outer query Many of the previous queries can be specified in an alternative form using nesting Query 1: Retrieve the name and address of all employees who work for the 'Research' department IM ISU Database 54 More Complex Queries (cont.) Q1: SELECT FROM WHERE IM ISU FNAME, LNAME, ADDRESS EMPLOYEE DNO IN (SELECT DNUMBER FROM DEPARTMENT WHERE DNAME='Research'); » The nested query selects the number of the 'Research' department » The outer query select an EMPLOYEE tuple if its DNO value is in the result of either nested query Database 55 More Complex Queries (cont.) » The comparison operator IN compares a value v with a set (or multi-set) of values V, and evaluates to TRUE if v is one of the elements in V » In general, we can have several levels of nested queries » In this example, the nested query is not correlated with the outer query A reference to an unqualified attribute refers to the relation declared in the innermost nested query IM ISU Database 56 More Complex Queries (cont.) Query 16: Retrieve the name of each employee who has a dependent with the same first name and same sex as the employee Q16: SELECT E.FNAME, E.LNAME FROM EMPLOYEE AS E WHERE E.SSN IN (SELECT ESSN FROM DEPENDENT WHERE E.FNAME=DEPENDENT_NAME AND E.SEX=SEX); IM ISU Database 57 More Complex Queries (cont.) Correlated Nested Queries If a condition in the WHERE-clause of a nested query references an attribute of a relation declared in the outer query, the two queries are said to be correlated The result of a correlated nested query is different for each tuple (or combination of tuples) of the relation(s) the outer query IM ISU Database 58 More Complex Queries (cont.) A query written with nested SELECT... FROM... WHERE... blocks and using the = or IN comparison operators can always be expressed as a single block query For example, Q16 may be written as in Q16A Q16A: SELECT E.FNAME, E.LNAME FROM EMPLOYEE AS E, DEPENDENT AS D WHERE E.SSN = D.ESSN AND E.SEX = D.SEX AND E.FNAME=D.DEPENDENT_NAME; IM ISU Database 59 More Complex Queries (cont.) The EXISTS Function EXISTS is used to check whether the result of a correlated nested query is empty or not We can formulate Query 16 in an alternative form that uses EXISTS as Q16B below IM ISU Database 60 More Complex Queries (cont.) Q16: SELECT E.FNAME, E.LNAME FROM EMPLOYEE AS E WHERE EXISTS (SELECT * FROM DEPENDENT WHERE E.SSN = ESSN AND E.SEX = SEX AND E.FNAME = DEPENDENT_NAME); IM ISU Database 61 More Complex Queries (cont.) Query 6: Retrieve the names of employees who have no dependents Q6: SELECT FNAME, LNAME FROM EMPLOYEE WHERE NOT EXISTS (SELECT * FROM DEPENDENT WHERE SSN=ESSN); » In Q6, the correlated nested query retrieves all DEPENDENT tuples related to an EMPLOYEE » If none exist, EMPLOYEE tuple is selected IM ISU Database 62 More Complex Queries (cont.) Explicit Sets It is also possible to use an explicit (enumerated) set of values in the WHEREclause rather than a nested query Query 17: Retrieve the social security numbers of all employees who work on project number 1, 2, or 3 Q13: SELECT FROM WHERE IM ISU DISTINCT ESSN WORKS_ON PNO IN (1, 2, 3); Database 63 More Complex Queries (cont.) NULLS in SQL Queries SQL allows queries that check if a value is NULL (missing or undefined or not applicable) Any comparison with null returns unknown » E.g. 5 < null or null <> null or IM ISU Database null = null 64 More Complex Queries (cont.) Three-valued logic using the truth value unknown: OR: (unknown or true) = true, (unknown or false) = unknown, (unknown or unknown) = unknown AND: (true and unknown) = unknown, (false and unknown) = false, (unknown and unknown) = unknown NOT: (not unknown) = unknown “P is unknown” evaluates to true if predicate P evaluates to unknown IM ISU Database 65 More Complex Queries (cont.) SQL uses IS or IS NOT to compare NULLs because it considers each NULL value distinct from other NULL values Query 18: Retrieve the names of all employees who do not have supervisors Q18: SELECT FROM WHERE » Result of Q18 IM ISU FNAME, LNAME EMPLOYEE SUPERSSN IS NULL; Database 66 More Complex Queries (cont.) Aggregate Functions Include COUNT, SUM, MAX, MIN, and AVG Query 19: Find the sum of salaries, the maximum salary, the minimum salary, and the average salary among all employees Q19: SELECT SUM(SALARY), MAX(SALARY), MIN(SALARY), AVG(SALARY) FROM EMPLOYEE; IM ISU Database 67 More Complex Queries (cont.) Queries 21 and 22: Retrieve the total number of employees in the company (Q21), and the number of employees in the 'Research' department (Q22) Q21: SELECT FROM COUNT (*) EMPLOYEE; Q22: SELECT COUNT (*) FROM WHERE IM ISU EMPLOYEE, DEPARTMENT DNO=DNUMBER AND DNAME='Research’; Database 68 More Complex Queries (cont.) Grouping IM ISU In many cases, we want to apply the aggregate functions to subgroups of tuples in a relation Each subgroup consists of the tuples having the same value for the grouping attribute(s) The function is applied to each subgroup independently SQL has a GROUP BY-clause for specifying the grouping attributes, which must also appear in the SELECT-clause Database 69 More Complex Queries (cont.) Query 24: For each department, retrieve the department number, the number of employees in the department, and their average salary Q24: SELECT DNO, COUNT (*), AVG (SALARY) FROM EMPLOYEE GROUP BY DNO; » In Q24, the EMPLOYEE are divided into groups having the same value for attribute DNO » The COUNT and AVG functions are applied to each subgroup of tuples separately IM ISU Database 70 More Complex Queries (cont.) Illustration of Q24 IM ISU Database 71 More Complex Queries (cont.) Query 25: For each project, retrieve the project number, project name, and the number of employees working on that project Q25: SELECT PNUMBER, PNAME, COUNT (*) FROM PROJECT, WORKS_ON WHERE PNUMBER=PNO GROUP BY PNUMBER, PNAME; » In this case, the grouping and functions are applied after the joining of the two relations IM ISU Database 72 More Complex Queries (cont.) The HAVING-CLAUSE Sometimes we want to retrieve the values of functions for only those groups that satisfy certain conditions The HAVING-clause is used for specifying a selection condition on groups (rather than on individual tuples) IM ISU Database 73 More Complex Queries (cont.) Query 26: For each project on which more than two employees work, retrieve the project number, project name, and the number of employees Q26: SELECT PNUMBER, PNAME, COUNT (*) FROM PROJECT, WORKS_ON WHERE PNUMBER=PNO GROUP BY PNUMBER, PNAME HAVING COUNT (*) > 2; IM ISU Database 74 More Complex Queries (cont.) Illustration of Query 26 IM ISU Database 75 More Complex Queries (cont.) Illustration of Query 26 IM ISU Database 76 More Complex Queries (cont.) Summary of SQL Queries A query in SQL can consist of up to six clauses, but only the first two, SELECT and FROM, are mandatory, ordered as follows SELECT <attribute list> FROM <table list> [WHERE <condition>] [GROUP BY <grouping attribute(s)>] [HAVING <group condition>] [ORDER BY <attribute list>] IM ISU Database 77 More Complex Queries (cont.) A query is evaluated by » first applying the FROM-clause, » followed by the WHERE-clause, » then GROUP BY and HAVING, and » finally the SELECT-clause IM ISU Database 78 Specifying Updates in SQL SQL commands to modify the database INSERT, DELETE, and UPDATE INSERT Command In its simplest form, it is used to add one or more tuples to a relation Attribute values should be listed in the same order as the attributes were specified in the CREATE TABLE command IM ISU Database 79 Specifying Updates in SQL (cont.) Example: Add a new tuple to the EMPLOYEE relation U1: IM ISU INSERT INTO EMPLOYEE VALUES ('Richard','K','Marini', '653298653', '30-DEC-52', '98 Oak Forest,Katy,TX', 'M', 37000,'987654321', 4); Database 80 Specifying Updates in SQL (cont.) An alternate form of INSERT specifies explicitly the attribute names that correspond to the values in the new tuple Attributes with NULL values can be left out Example: Insert a tuple for a new EMPLOYEE for whom we only know the FNAME, LNAME, and SSN attributes U1A: INSERT INTO EMPLOYEE (FNAME, LNAME, SSN) VALUES ('Richard', 'Marini', '653298653') IM ISU Database 81 Specifying Updates in SQL (cont.) Note: Only the constraints specified in the DDL commands are automatically enforced by the DBMS when updates are applied to the database If a system does not support some constraint, the users must enforce the constraint U2: INSERT INTO EMPLOYEE (FNAME, LNAME, SSN, DNO) VALUES (‘Robert’, ‘Hatcher’, ‘980760540’, 2); (* U2 is rejected if referential integrity checking is provided by DBMS *) IM ISU Database 82 Specifying Updates in SQL (cont.) A DBMS enforcing NOT NULL will reject an INSERT command in which an attribute declared to be NOT NULL does not have a value U2A: INSERT INTO EMPLOYEE (FNAME, LNAME, DNO) VALUES (‘Robert’, ‘Hatcher’, 5); (* U2A is rejected if NOT NULL checking is provided by DBMS *) IM ISU Database 83 Specifying Updates in SQL (cont.) Another variation of INSERT allows insertion of multiple tuples resulting from a query into a relation Example: Suppose we want to create a temporary table that has the name, number of employees, and total salaries for each department. A table DEPTS_INFO is created by U3A, and is loaded with the summary information retrieved from the database by the query in U3B IM ISU Database 84 Specifying Updates in SQL (cont.) U3A: CREATE TABLE DEPTS_INFO (DEPT_NAME VARCHAR(10), NO_OF_EMPS INTEGER, TOTAL_SAL INTEGER); U3B: INSERT INTO DEPTS_INFO (DEPT_NAME, NO_OF_EMPS, TOTAL_SAL) SELECT DNAME, COUNT (*), SUM (SALARY) FROM DEPARTMENT, EMPLOYEE WHERE DNUMBER=DNO GROUP BY DNAME ; IM ISU Database 85 Specifying Updates in SQL (cont.) DELETE Command Removes tuples from a relation Includes a WHERE-clause to select the tuples to be deleted Tuples are deleted from only one table at a time (unless CASCADE is specified on a referential integrity constraint) A missing WHERE-clause specifies that all tuples in the relation are to be deleted IM ISU Database 86 Specifying Updates in SQL (cont.) Examples: U4A: DELETE FROM EMPLOYEE WHERE LNAME='Brown’ U4B: DELETE FROM EMPLOYEE WHERE SSN='123456789’ U4C: DELETE FROM EMPLOYEE WHERE DNO IN (SELECT DNUMBER FROM DEPARTMENT WHERE DNAME='Research') U4D: DELETE FROM EMPLOYEE IM ISU Database 87 Specifying Updates in SQL (cont.) UPDATE Command Used to modify attribute values of one or more selected tuples A WHERE-clause selects the tuples to be modified An additional SET-clause specifies the attributes to be modified and their new values Each command modifies tuples in the same relation Referential integrity should be enforced IM ISU Database 88 Specifying Updates in SQL (cont.) Example: Change the location and controlling department number of project number 10 to 'Bellaire' and 5, respectively. U5: UPDATE PROJECT SET PLOCATION = 'Bellaire', DNUM = 5 WHERE PNUMBER=10 Example: Give all employees in the 'Research' department a 10% raise in salary. IM ISU Database 89 Specifying Updates in SQL (cont.) U6: UPDATE EMPLOYEE SET SALARY = SALARY *1.1 WHERE DNO IN (SELECT DNUMBER FROM DEPARTMENT WHERE DNAME='Research') » In this request, the modified SALARY value depends on the original SALARY value in each tuple IM ISU Database 90 Relational Views in SQL Basic concepts A view is a single virtual table that is derived from other tables, which could be base tables or previously defined views A view does not necessarily exist in physical form; that means the view does not store any tuples There are limitations on possible update operations that can be applied to views There is no limitation on querying a view IM ISU Database 91 Relational Views in SQL (cont.) Specify Views Using CREATE VIEW command The view attribute names can be inherited from the attribute names of the tables in the defining query Examples: V1:CREATE SELECT FROM WHERE IM ISU VIEW WORKS_ON1 AS FNAME, LNAME, PNAME, HOURS EMPLOYEE, PROJECT, WORKS_ON SSN=ESSN AND PNO=PNUMBER; Database 92 Relational Views in SQL (cont.) V2:CREATE VIEW DEPT_INFO (DEPT_NAME, NO_OF_EMPS, TOTAL_SAL) AS SELECT DNAME, COUNT (*), SUM (SALARY) FROM DEPARTMENT, EMPLOYEE WHERE DNUMBER=DNO GROUP BY DNAME; IM ISU Database 93 Relational Views in SQL (cont.) Queries on Views A view can be defined to simplify frequently occurring queries Example: Retrieve the last name and first name of all employees who work on 'ProjectX'. QV1: SELECT PNAME, FNAME, LNAME FROM WORKS_ON1 WHERE PNAME='ProjectX'; » Without the view WORKS_ON1, this query specification would require two join conditions IM ISU Database 94 Relational Views in SQL (cont.) A view is always up-to-date » If the base tables on which the view is defined are modified, the DBMS is responsible for keeping it up to date » Hence, the view is not realized at the time of view definition, but rather at the time we specify a query on the view A view is removed using the DROP VIEW command » Example: V1A: DROP VIEW WORKS_ON1; IM ISU Database 95 Relational Views in SQL (cont.) Updating of Views A view update operation may be mapped in multiple ways to update operations on the defining base relations The topic of updating views is still an active research area Example: Suppose we issue the command in UV1 to update the WORKS_ON1 view by modifying the PNAME attribute of 'John Smith' from 'ProductX' to 'ProductY'. IM ISU Database 96 Relational Views in SQL (cont.) UV1: UPDATE SET WHERE WORKS_ON1 PNAME = 'ProductY' LNAME='Smith' AND FNAME='John' AND PNAME='ProductX’ » This can be mapped into several updates on the base relations to give the desired update on the view » Two possibilities 1. Change the name of the 'ProductX' tuple in the PROJECT relation to 'ProductY’ 2. Relate 'John Smith' to the 'ProductY' PROJECT tuple in place of the 'ProductX' PROJECT tuple IM ISU Database 97 Relational Views in SQL (cont.) » Method 1 UPDATE PROJECT SET PNAME = 'ProductY' WHERE PNAME = 'ProductX’ – This has the effect of changing all the view tuples with PNAME = ‘ProductX’ – It is quite unlikely that the user who specified view update UV1 wants the update to be interpreted this way IM ISU Database 98 Relational Views in SQL (cont.) » Method 2 UPDATE WORKS_ON SET PNO = (SELECT PNUMBER FROM PROJECT WHERE PNAME='ProductY') WHERE ESSN IN (SELECT SSN FROM EMPLOYEE WHERE LNAME='Smith' AND FNAME='John') AND PNO IN (SELECT PNUMBER FROM PROJECT IM ISU Database WHERE PNAME='ProductX')99 Relational Views in SQL (cont.) Some view updates may not make much sense » Example: Modify the TOTAL_SAL attribute of DEPT_INFO as in UV2 UV2: UPDATE DEPT_INFO SET TOTAL_SAL=100000 WHERE DNAME='Research'; In general, we cannot guarantee that any view can be updated A view update is feasible only if one update on the base relations can accomplish the desired update effect on the view IM ISU Database 100 Relational Views in SQL (cont.) If a view update can be mapped to more than one update on the underlying base relations, we have to choose the desired update General guideline » A view with a single defining table is updatable if the view attributes contain the primary key » Views defined on multiple tables using joins are generally not updatable » Views defined aggregate functions are not updatable IM ISU Database 101