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Chapter 8
  • SQL-99: Schema Definition, Basic Constraints, and Queries
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Data Definition, Constraints, and Schema Changes
  • Used to CREATE, DROP, and ALTER the descriptions of the tables (relations) of a database
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CREATE TABLE
  • Specifies a new base relation by giving it a name, and specifying each of its attributes and their data types (INTEGER, FLOAT, DECIMAL(i,j), CHAR(n), VARCHAR(n))
  • 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)  );
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CREATE TABLE
  • In SQL2, can use the CREATE TABLE command for specifying the primary key attributes, secondary keys, and referential integrity constraints (foreign keys).
  • Key attributes can be specified via the PRIMARY KEY and UNIQUE phrases
    • 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  );
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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
  • Example:

    DROP TABLE  DEPENDENT;
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ALTER TABLE
  • Used to add an attribute to one of the base relations
  • The new attribute will have NULLs in all the tuples of the relation right after the command is executed; hence, the NOT NULL constraint is not allowed  for such an attribute
  • Example:

    ALTER TABLE  EMPLOYEE  ADD   JOB   VARCHAR(12);
  • 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.
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Features Added in SQL2 and SQL-99
  • CREATE SCHEMA
  • REFERENTIAL INTEGRITY OPTIONS
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CREATE SCHEMA
  • Specifies a new database schema by giving it a name
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REFERENTIAL INTEGRITY OPTIONS
  • We can specify RESTRICT, CASCADE, SET NULL or SET DEFAULT on referential integrity constraints (foreign keys)

    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
    ON DELETE SET DEFAULT ON UPDATE CASCADE  );
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REFERENTIAL INTEGRITY OPTIONS (continued)
    • CREATE TABLE   EMP
      ( ENAME VARCHAR(30) NOT NULL,
      ESSN CHAR(9),
      BDATE DATE,
      DNO INTEGER  DEFAULT 1,
      SUPERSSN CHAR(9),
      PRIMARY KEY (ESSN),
      FOREIGN KEY (DNO) REFERENCES DEPT
                ON DELETE SET DEFAULT ON UPDATE CASCADE,
      FOREIGN KEY (SUPERSSN) REFERENCES EMP
                ON DELETE SET NULL ON UPDATE CASCADE  );
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Additional Data Types in SQL2 and SQL-99
  • Has DATE, TIME, and TIMESTAMP data types
  • DATE:
    • Made up of year-month-day in the format yyyy-mm-dd
  • TIME:
    • Made up of hour:minute:second in the format 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
  • TIMESTAMP:
    • Has both DATE and TIME components
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Additional Data Types in SQL2 and SQL-99 (cont.)
  • INTERVAL:
    • Specifies a relative value rather than an absolute value
    • Can be DAY/TIME intervals or YEAR/MONTH intervals
    • Can be positive or negative when added to or subtracted from an absolute value, the result is an absolute value
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Retrieval Queries in SQL
  • 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
  • Important distinction between SQL and the formal relational model; SQL allows a table (relation) to have two or more tuples that are identical in all their attribute values
  • 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
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Retrieval Queries in SQL (cont.)
  • Basic form of the SQL SELECT statement is called a mapping  or a SELECT-FROM-WHERE block
    • 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
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Relational Database Schema--Figure 5.5
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Populated Database--Fig.5.6
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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
  • 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 and the WHERE-clause specifies the selection condition
    • However, the result of the query may contain  duplicate tuples
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Simple SQL Queries (cont.)
  • Query 1: Retrieve the name and address of all employees who work for the 'Research' department.
    • Q1: SELECT FNAME, LNAME, ADDRESS
      FROM EMPLOYEE, DEPARTMENT
      WHERE DNAME='Research' AND DNUMBER=DNO
    • Similar to a SELECT-PROJECT-JOIN sequence of relational algebra operations
    • (DNAME='Research') is a selection condition  (corresponds to a SELECT operation in relational algebra)
    • (DNUMBER=DNO) is a join condition (corresponds to a JOIN operation in relational algebra)
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Simple SQL Queries (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 PNUMBER, DNUM, LNAME, BDATE, ADDRESS
      FROM PROJECT, DEPARTMENT, EMPLOYEE
      WHERE DNUM=DNUMBER AND MGRSSN=SSN AND PLOCATION='Stafford'
    • 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
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Aliases, * and DISTINCT, Empty WHERE-clause
  • 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
  • Example:


  • EMPLOYEE.LNAME, DEPARTMENT.DNAME
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ALIASES
  • 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 E.FNAME, E.LNAME, S.FNAME, S.LNAME
    FROM EMPLOYEE E S
    WHERE E.SUPERSSN=S.SSN
    • In Q8, the alternate relation names E and S are called aliases  or tuple variables for the EMPLOYEE relation
    • We can think of E and S as two different copies  of EMPLOYEE; E represents employees in role of supervisees  and S represents employees in role of supervisors
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ALIASES (cont.)
    • Aliasing can also be used in any SQL query for convenience
      Can also use the AS keyword to specify aliases

      Q8: SELECT E.FNAME, E.LNAME, S.FNAME, S.LNAME
      FROM EMPLOYEE AS E, EMPLOYEE AS S
      WHERE E.SUPERSSN=S.SSN
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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 the condition WHERE TRUE
  • Query 9: Retrieve the SSN values for all employees.


    • Q9: SELECT SSN
      FROM EMPLOYEE
  • 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
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UNSPECIFIED
WHERE-clause (cont.)
  • Example:

    Q10: SELECT SSN, DNAME
    FROM EMPLOYEE, DEPARTMENT
    • It is extremely important not to overlook specifying any selection and join conditions in the WHERE-clause; otherwise, incorrect and very large relations may result
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USE OF *
  • To retrieve all the attribute values of the selected tuples, a * is used, which stands for all the attributes
    Examples:

  • Q1C: SELECT *
    FROM EMPLOYEE
    WHERE DNO=5

    Q1D: SELECT *
    FROM EMPLOYEE, DEPARTMENT
    WHERE DNAME='Research' AND DNO=DNUMBER
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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
  • For example, the result of Q11 may have duplicate SALARY values whereas Q11A does not have any duplicate values
  • Q11: SELECT SALARY
    FROM EMPLOYEE
    Q11A: SELECT DISTINCT SALARY
    FROM EMPLOYEE
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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; duplicate tuples are eliminated from the result
  • The set operations apply only to union compatible relations ; the two relations must have the same attributes and the attributes must appear in the same order
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SET OPERATIONS (cont.)
  • 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.

    Q4: (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')
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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.

    Q1: SELECT FNAME, LNAME, ADDRESS
    FROM EMPLOYEE
    WHERE DNO IN  (SELECT  DNUMBER
    FROM DEPARTMENT
    WHERE DNAME='Research' )
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NESTING OF QUERIES (cont.)
  • 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
  • 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
  • A reference to an unqualified attribute  refers to the relation declared in the innermost nested query
  • In this example, the nested query is not correlated  with the outer query
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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
  • Query 12: Retrieve the name of each employee who has a dependent with the same first name as the employee.

    Q12: SELECT  E.FNAME, E.LNAME
    FROM EMPLOYEE AS E
    WHERE E.SSN IN (SELECT ESSN
    FROM DEPENDENT
    WHERE ESSN=E.SSN AND
    E.FNAME=DEPENDENT_NAME)
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CORRELATED NESTED QUERIES (cont.)
    • In Q12, the nested query has a different result for each tuple  in the outer query
    • 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, Q12 may be written as in Q12A

      Q12A: SELECT E.FNAME, E.LNAME
      FROM EMPLOYEE E, DEPENDENT D
      WHERE E.SSN=D.ESSN AND E.FNAME=D.DEPENDENT_NAME
    • The original SQL as specified for SYSTEM R also had a CONTAINS comparison operator, which is used in conjunction with nested correlated queries
    • This operator was dropped from the language, possibly because of the difficulty in implementing it efficiently
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CORRELATED NESTED QUERIES (cont.)
    • Most implementations of SQL do not  have this operator
    • The CONTAINS operator compares two sets of values , and returns TRUE if one set contains all values in the other set
        (reminiscent of the division operation of algebra).
      • Query 3: Retrieve the name of each employee who works on all  the projects controlled by department number 5.

        Q3: SELECT FNAME, LNAME
        FROM EMPLOYEE
        WHERE  ( (SELECT PNO
           FROM WORKS_ON
           WHERE SSN=ESSN)
           CONTAINS
          (SELECT PNUMBER
           FROM PROJECT
           WHERE DNUM=5) )
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CORRELATED NESTED QUERIES (cont.)
        • In Q3, the second nested query, which is not correlated with the outer query, retrieves the project numbers of all projects controlled by department 5
        • The first nested query, which is correlated, retrieves the project numbers on which the employee works, which is different for each employee tuple  because of the correlation
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THE EXISTS FUNCTION
  • EXISTS is used to check whether the result of a correlated nested query is empty (contains no tuples) or not
  • We can formulate Query 12 in an alternative form that uses EXISTS as Q12B below
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THE EXISTS FUNCTION (cont.)
  • Query 12: Retrieve the name of each employee who has a dependent with the same first name as the employee.

    Q12B: SELECT  FNAME, LNAME
    FROM EMPLOYEE
    WHERE EXISTS  (SELECT *
    FROM DEPENDENT
    WHERE SSN=ESSN AND FNAME=DEPENDENT_NAME)
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THE EXISTS FUNCTION (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 tuple. If none exist , the EMPLOYEE tuple is selected
    • EXISTS is necessary for the expressive power of SQL
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EXPLICIT SETS
  • It is also possible to use an explicit (enumerated) set of values in the WHERE-clause rather than a nested query
  • Query 13: Retrieve the social security numbers of all employees who work on project number 1, 2, or 3.


    • Q13: SELECT  DISTINCT ESSN
      FROM WORKS_ON
      WHERE PNO IN  (1, 2, 3)
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NULLS IN SQL QUERIES
  • SQL allows queries that check if a value is NULL (missing or undefined or not applicable)
  • SQL uses IS or IS NOT to compare NULLs because it considers each NULL value distinct from other NULL values, so equality comparison is not appropriate .
  • Query 14: Retrieve the names of all employees who do not have supervisors.
    Q14: SELECT  FNAME, LNAME
    FROM EMPLOYEE
    WHERE SUPERSSN  IS  NULL
    Note: If a join condition is specified, tuples with NULL values for the join attributes are not included in the result
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Joined Relations Feature
in SQL2
  • Can specify a "joined relation" in the FROM-clause
  • Looks like any other relation but is the result of a join
  • Allows the user to specify different types of joins (regular "theta" JOIN, NATURAL JOIN, LEFT OUTER JOIN, RIGHT OUTER JOIN, CROSS JOIN, etc)
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Joined Relations Feature
in SQL2 (cont.)
  • Examples:

    Q8: SELECT E.FNAME, E.LNAME, S.FNAME, S.LNAME
    FROM EMPLOYEE E S
    WHERE E.SUPERSSN=S.SSN

    can be written as:

    Q8: SELECT E.FNAME, E.LNAME, S.FNAME, S.LNAME
    FROM (EMPLOYEE E LEFT OUTER JOIN EMPLOYEES
    ON  E.SUPERSSN=S.SSN)

    Q1: SELECT FNAME, LNAME, ADDRESS
    FROM EMPLOYEE, DEPARTMENT
    WHERE DNAME='Research' AND DNUMBER=DNO
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Joined Relations Feature
in SQL2 (cont.)
  • could be written as:

    Q1: SELECT FNAME, LNAME, ADDRESS
    FROM (EMPLOYEE JOIN DEPARTMENT
    ON DNUMBER=DNO)
    WHERE DNAME='Research’

    or as:

    Q1: SELECT FNAME, LNAME, ADDRESS
    FROM (EMPLOYEE NATURAL JOIN DEPARTMENT
    AS DEPT(DNAME, DNO, MSSN, MSDATE)
    WHERE DNAME='Research’
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Joined Relations Feature
in SQL2 (cont.)
  • Another Example;
    • Q2 could be written as follows; this illustrates multiple joins in the joined tables

      Q2: SELECT PNUMBER, DNUM, LNAME, BDATE, ADDRESS
      FROM (PROJECT JOIN DEPARTMENT ON DNUM=DNUMBER) JOIN EMPLOYEE ON MGRSSN=SSN) )
      WHERE PLOCATION='Stafford’
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AGGREGATE FUNCTIONS
  • Include COUNT, SUM, MAX, MIN, and AVG
  • Query 15: Find the maximum salary, the minimum salary, and the average salary among all employees.

    Q15: SELECT  MAX(SALARY), MIN(SALARY), AVG(SALARY)
    FROM EMPLOYEE
    • Some SQL implementations may not allow more than one function  in the SELECT-clause
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AGGREGATE FUNCTIONS (cont.)
  • Query 16: Find the maximum salary, the minimum salary, and the average salary among employees who work for the 'Research' department.

    Q16: SELECT MAX(SALARY), MIN(SALARY), AVG(SALARY)
    FROM EMPLOYEE, DEPARTMENT
    WHERE DNO=DNUMBER AND DNAME='Research'
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AGGREGATE FUNCTIONS (cont.)
  • Queries 17 and 18: Retrieve the total number of employees in the company (Q17), and the number of employees in the 'Research' department (Q18).

    Q17: SELECT  COUNT (*)
    FROM EMPLOYEE

    Q18: SELECT  COUNT (*)
    FROM EMPLOYEE, DEPARTMENT
    WHERE DNO=DNUMBER AND DNAME='Research’
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GROUPING
  • In many cases, we want to apply the aggregate functions to subgroups of tuples in a relation
  • Each subgroup of tuples consists of the set of tuples that have 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
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GROUPING (cont.)
  • Query 20: For each department, retrieve the department number, the number of employees in the department, and their average salary.

    Q20: SELECT DNO, COUNT (*), AVG (SALARY)
    FROM EMPLOYEE
    GROUP BY DNO
    • In Q20, the EMPLOYEE tuples are divided into groups--each group having the same value for the grouping attribute DNO
    • The COUNT and AVG functions are applied to each such group of tuples separately
    • The SELECT-clause includes only the grouping attribute and the functions to be applied on each group of tuples
    • A join condition can be used in conjunction with grouping
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GROUPING (cont.)
  • Query 21: For each project, retrieve the project number, project name, and the number of employees who work on that project.

    Q21: 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
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THE HAVING-CLAUSE
  • Sometimes we want to retrieve the values of these 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)
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THE HAVING-CLAUSE (cont.)
  • Query 22: For each project on which more than two employees work , retrieve the project number, project name, and the number of employees who work on that project.

    Q22:     SELECT PNUMBER, PNAME, COUNT (*)
    FROM PROJECT, WORKS_ON
    WHERE PNUMBER=PNO
    GROUP BY PNUMBER, PNAME
    HAVING COUNT (*) > 2
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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, and '_' replaces a single arbitrary character
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SUBSTRING COMPARISON (cont.)
  • Query 25:  Retrieve all employees whose address is in Houston, Texas. Here, the value of the ADDRESS attribute must contain the substring 'Houston,TX'.

    Q25: SELECT FNAME, LNAME
    FROM EMPLOYEE
    WHERE ADDRESS LIKE '%Houston,TX%’
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SUBSTRING COMPARISON (cont.)
  • Query 26: Retrieve all employees who were born during the 1950s. Here, '5' must be the 8th character of the string (according to our format for date), so the BDATE value is '_______5_', with each underscore as a place holder for a single arbitrary character.

    Q26: SELECT FNAME, LNAME
    FROM EMPLOYEE
    WHERE BDATE LIKE '_______5_’
  • The LIKE operator allows us to get around the fact that each value is considered atomic and indivisible; hence, in SQL, character string attribute values are not atomic
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ARITHMETIC OPERATIONS
  • The standard arithmetic operators '+', '-'. '*', and '/' (for addition, subtraction, multiplication, and division, respectively) can be applied to numeric values in an SQL query result
  • Query 27: Show the effect of giving all employees who work on the 'ProductX' project a 10% raise.

    Q27: SELECT FNAME, LNAME, 1.1*SALARY
    FROM EMPLOYEE, WORKS_ON, PROJECT
    WHERE SSN=ESSN AND PNO=PNUMBER AND PNAME='ProductX’
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ORDER BY
  • The ORDER BY clause is used to sort the tuples in a query result based on the values of some attribute(s)
  • Query 28: 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.

    Q28: SELECT DNAME, LNAME, FNAME, PNAME
          FROM DEPARTMENT, EMPLOYEE, WORKS_ON, PROJECT
    WHERE DNUMBER=DNO AND SSN=ESSN AND PNO=PNUMBER
    ORDER BY DNAME, LNAME
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ORDER BY (cont.)
  • The default order is in ascending order of values
  • We can specify the keyword DESC if we want a descending order; the keyword ASC can be used to explicitly specify ascending order, even though it is the default
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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. The clauses are specified in the following order:

    SELECT <attribute list>
    FROM <table list>
    [WHERE <condition>]
    [GROUP BY <grouping attribute(s)>]
    [HAVING <group condition>]
    [ORDER BY <attribute list>]
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Summary of SQL Queries (cont.)
  • The SELECT-clause lists the attributes or functions to be retrieved
  • The FROM-clause specifies all relations (or aliases) needed in the query but not those needed in nested queries
  • The WHERE-clause specifies the conditions for selection and join of tuples from the relations specified in the FROM-clause
  • GROUP BY specifies grouping attributes
  • HAVING specifies a condition for selection of groups
  • ORDER BY specifies an order for displaying the result of a query
  • A query is evaluated by first applying the WHERE-clause, then GROUP BY and HAVING, and finally the SELECT-clause
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Specifying Updates in SQL
  • There are three SQL commands to modify the database; INSERT, DELETE, and UPDATE
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INSERT
  • 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
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INSERT (cont.)
  • Example:

    U1: INSERT INTO  EMPLOYEE
    VALUES ('Richard','K','Marini', '653298653', '30-DEC-52',
    '98 Oak Forest,Katy,TX', 'M', 37000,'987654321', 4 )
  • 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')
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INSERT (cont.)
  • Important Note: Only the constraints specified in the DDL commands are automatically enforced by the DBMS when updates are applied to the database
  • Another variation of INSERT allows insertion of multiple tuples  resulting from a query into a relation
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INSERT (cont.)
    • 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.

      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 ;
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INSERT (cont.)
  • Note: The DEPTS_INFO table may not be up-to-date if we change the tuples in either the DEPARTMENT or the EMPLOYEE relations after  issuing U3B. We have to create a view (see later) to keep such a table up to date.
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DELETE
  • 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; the table then becomes an empty table
  • The number of tuples deleted depends on the number of tuples in the relation that satisfy the WHERE-clause
  • Referential integrity should be enforced
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DELETE (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
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UPDATE
  • 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
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UPDATE (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
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UPDATE (cont.)
  • Example: Give all employees in the 'Research' department a 10% raise in salary.

    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
  • The reference to the SALARY attribute on the right of = refers to the old SALARY value before modification
  • The reference to the SALARY attribute on the left of = refers to the new SALARY value after modification