SQL: SchemaDefinition, Constraints, and Queries and Views
History of SQL
SQL: Structured Query Language In 1974, D.
Chamberlin (IBM Laboratory) defined language called English Query Language’ (SEQUEL).
San Jose ‘Structured A revised version, SEQUEL/2, was defined in 1976 but name was subsequently changed to SQL for legal reasons.
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History of SQL
Still pronounced pronunciation is ‘see-quel’, ‘S-Q-L’.
though official IBM subsequently produced a prototype DBMS called
System R
, based on SEQUEL/2.
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History of SQL
In late 70s, ORACLE appeared and was probably first commercial RDBMS based on SQL.
In 1987, ANSI and ISO published an initial standard for SQL.
In 1989, ISO published an addendum that defined an ‘Integrity Enhancement Feature’.
In 1992, first major revision to ISO standard occurred, referred to as SQL2 or SQL/92.
In 1999, SQL:1999 was released with support for object-oriented data management.
In late 2003, SQL:2003 was released.
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DBMS Languages
Data Definition Language (DDL) Data Manipulation Language (DML) High-Level or Non-procedural Languages: These include the relational language SQL May be used in a standalone or may be embedded in a programming language Low Level or Procedural Languages: These must be embedded in a programming language
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DBMS Languages
Data Definition Language (DDL):
Used by the DBA and database designers to specify the conceptual schema of a database.
In many DBMSs, the DDL is also used to define internal and external schemas (views).
In some DBMSs, separate
storage definition language (SDL)
and
view definition language (VDL)
are used to define internal and external schemas.
SDL is typically realized via DBMS commands provided to the DBA and database designers
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DBMS Languages
Data Manipulation Language (DML):
Used to specify database retrievals and updates DML commands can be
embedded
in a general purpose programming language (host language), such as C++, or Java.
A library of functions can also be provided to access the DBMS from a programming language Alternatively, stand-alone DML commands can be applied directly (called a
query language
).
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Types of DML
High Level or Non-procedural Language:
For example, the SQL relational language Are “set”-oriented and specify what data to retrieve rather than how to retrieve it. Also called
declarative
languages.
Low Level or Procedural Language:
Retrieve data one record-at-a-time; Constructs such as looping are needed to retrieve multiple records, along with positioning pointers.
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Objectives of SQL
Ideally, database language should allow user to: create the database and relation structures; perform insertion, modification, deletion of data from relations; perform simple and complex queries.
It must be portable.
SQL is relatively easy to learn: it is non-procedural - you specify require, rather than
how
to get it
what
information you Can be used by range of users including DBAs, management, application developers, and other types of end users.
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Objectives of SQL
Consists of standard English words: 1) CREATE TABLE Staff( staffNo VARCHAR(5), lName VARCHAR(15), salary INTEGER); 2) INSERT INTO Staff VALUES ( ‘SG16’, ‘Brown’, 8300); 3) SELECT staffNo, lName, salary FROM Staff WHERE salary > 10000; Slide 8 11
All non-numeric literals must be enclosed in single quotes (e.g.
‘London’).
All numeric literals must not be enclosed in quotes (e.g. 650.00).
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Attribute Data Types in SQL
Basic
data types
Numeric
data types • Integer numbers: INTEGER and SMALLINT • Floating-point (real) numbers: FLOAT or REAL , and DOUBLE PRECISION
Character-string
data types • Fixed length: CHAR(
n
) • Varying length : VARCHAR(
n
)
Attribute Data Types in SQL (cont ’d.)
Bit-string
data types • Fixed length: BIT(
n
) • Varying length: BIT VARYING(
n
)
Boolean
data type • Values of TRUE or FALSE or NULL
DATE
data type • • Ten positions Components are YEAR , MONTH , and DAY in the form YYYY-MM-DD
Attribute Data Types in SQL (cont ’d.)
Additional data types
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
data type ( TIMESTAMP ) • Includes the DATE and TIME fields • • Plus a minimum of six positions for decimal fractions of seconds Optional WITH TIME ZONE qualifier
Attribute Data Types in SQL (cont ’d.)
Additional data types
INTERVAL
data type • • Specifies a relative value that can be used to increment or decrement an absolute value of a date, time, or timestamp Can be DAY/TIME intervals or YEAR/MONTH intervals
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
CREATE TABLE command can be used for specifying the primary key attributes, secondary key, and referential integrity constraints (foreign keys).
Basic constraints: Key and referential integrity constraints Restrictions on attribute domains and NULLs Constraints on individual tuples within a relation
Specifying Attribute Constraints and Attribute Defaults
NOT NULL NULL is not permitted for a particular attribute Default value
DEFAULT
CHECK
clause Dnumber INT NOT NULL CHECK (Dnumber > 0 AND Dnumber < 21);
Specifying Key and Referential Integrity Constraints
PRIMARY KEY
clause Specifies one or more attributes that make up the primary key of a relation Dnumber INT PRIMARY KEY;
Specifying Key and Referential Integrity Constraints (cont ’d.)
FOREIGN KEY
clause Default operation: reject update on violation Attach
referential triggered action
clause • • Options include RESTRICT , SET NULL , CASCADE , and SET DEFAULT CASCADE option suitable for “relationship” relations
REFERENTIAL INTEGRITY OPTIONS
CREATE TABLE DEPT ( DNAME VARCHAR(10) DNUMBER INTEGER NOT NULL, NOT NULL, MGRSSN CHAR(9) DEFAULT ‘NO’, MGRSTARTDATE CHAR(9), PRIMARY KEY (DNUMBER), UNIQUE (DNAME), FOREIGN KEY (MGRSSN) REFERENCES EMP ON DELETE SET DEFAULT ON UPDATE CASCADE); Slide 8 26
REFERENTIAL INTEGRITY OPTIONS (continued)
CREATE TABLE EMP( ENAME VARCHAR(30) ESSN BDATE CHAR(9), DATE, NOT NULL, DNO INTEGER DEFAULT 1, SUPERSSN CASCADE, CHAR(9), PRIMARY KEY (ESSN), FOREIGN KEY (DNO) REFERENCES DEPT ON DELETE SET DEFAULT ON UPDATE FOREIGN KEY (SUPERSSN) REFERENCES EMP ON DELETE SET NULL ON UPDATE CASCADE); Slide 8 27
Giving Names to Constraints
Keyword
CONSTRAINT
Name a constraint Useful for later altering
Specifying Constraints on Tuples Using CHECK
CHECK clauses at the end of a CREATE TABLE statement Apply to each tuple individually CHECK (Dept_create_date <= Mgr_start_date);
The DROP Command
DROP command Used to remove named schema elements, such as tables, domains, or constraint Drop behavior options: CASCADE and RESTRICT Examples:
DROP TABLE DEPENDENT; DROP SCHEMA COMPANY CASCADE;
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The ALTER Command
Alter table actions
include: Adding or dropping a column (attribute) The new attribute will have NULLs in all the tuples of the relation right after the command is executed the NOT NULL constraint is not allowed for such an attribute Changing a column definition Adding or dropping table constraints Examples: ALTER TABLE COMPANY.EMPLOYEE ADD COLUMN Job VARCHAR(12);
Or:
ALTER TABLE EMPLOYEE ADD JOB VARCHAR(12);
The ALTER Command (cont ’d.)
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.
To drop a column Choose either CASCADE or RESTRICT Change constraints specified on a table Add or drop a named constraint
ALTER TABLE Examples
ALTER TABLE
EMPLOYEE
ADD
JOB VARCHAR(12);
ALTER TABLE
EMPLOYEE
DROP
ADDRESS
CASCADE;
ALTER TABLE
DEPARTMENT
ALTER
MGRSSN
DROP DEFAULT;
ALTER TABLE
DEPARTMENT
ALTER
MGRSSN
SET DEFAULT
"333445555";
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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 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 (contd.)
Basic form of the SQL SELECT statement is called a
mapping
or a SELECT-FROM-WHERE
block
SELECT FROM WHERE
is a list of attribute names whose values are to be retrieved by the query
is a list of the relation names required to process the query is a conditional (Boolean) expression that identifies the tuples to be retrieved by the query Logical comparison operators: =, <, <=, >, >=, and <>
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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 following operations of the relational algebra: SELECT PROJECT JOIN All subsequent examples use the COMPANY database
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Simple SQL Queries (contd.)
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 FROM WHERE BDATE, ADDRESS EMPLOYEE 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 (contd.)
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 (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 (contd.)
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 PNUMBER, DNUM, LNAME, BDATE, ADDRESS PROJECT, DEPARTMENT, EMPLOYEE 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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Ambiguous Attribute Names
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
Declare alternative relation names 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 E.FNAME, E.LNAME, S.FNAME, S.LNAME
EMPLOYEE E S 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 (contd.)
Aliasing can also be used in any SQL query for convenience Can also use the AS keyword to specify aliases Q8: SELECT FROM WHERE E.FNAME, E.LNAME, S.FNAME, S.LNAME
EMPLOYEE AS E, EMPLOYEE AS S E.SUPERSSN=S.SSN
Renaming of Attributes Rename any attribute that appears in the result of a query
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UNSPECIFIED WHERE-clause
A
missing WHERE-clause
indicates no condition; 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 FROM SSN 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 (contd.)
Example: Q10: SELECT FROM SSN, DNAME 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 ASTERISK *
To retrieve all the attribute values of the selected tuples, a * is used, which stands for
all the attributes
Examples: Q1C: Q1D: SELECT FROM WHERE SELECT FROM WHERE * EMPLOYEE DNO=5 * EMPLOYEE, DEPARTMENT 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 FROM Q11A: SELECT FROM SALARY EMPLOYEE
DISTINCT
SALARY EMPLOYEE
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SET OPERATIONS
SQL has directly incorporated some set operations There is a union operation (UNION), set difference (EXCEPT) 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 (contd.)
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 FROM WHERE UNION (SELECT FROM WHERE DISTINCT PNUMBER PROJECT, DEPARTMENT, EMPLOYEE DNUM=DNUMBER AND MGRSSN=SSN AND LNAME='Smith') DISTINCT PNUMBER PROJECT, WORKS_ON, EMPLOYEE PNUMBER=PNO AND ESSN=SSN AND LNAME='Smith')
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NESTING OF QUERIES
A complete SELECT query, called a called the
outer query nested query
, can be specified within the WHERE-clause of another 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 FROM WHERE FROM WHERE FNAME, LNAME, ADDRESS EMPLOYEE DNO IN (SELECT DNUMBER DEPARTMENT DNAME='Research' )
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NESTING OF QUERIES (contd.)
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 evaluates to TRUE if v is one of the elements in V In general, we can have several levels of nested queries
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Nested Queries
Nested Queries (cont ’d.)
Use tuples of values in comparisons Place them within parentheses Other comparison operators
THE EXISTS FUNCTION AND CORRELATED NESTED QUERIES
Query 6: Retrieve the names of employees who have no dependents.
Q6: SELECT FROM WHERE FNAME, LNAME EMPLOYEE E NOT EXISTS (SELECT FROM * DEPENDENT WHERE E.SSN=ESSN) 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
In Q6, the correlated nested query retrieves all DEPENDENT tuples related to an EMPLOYEE tuple. If
none exist
, the EMPLOYEE tuple is selected
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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 FROM WHERE DISTINCT ESSN WORKS_ON 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.
Query 14: Retrieve the names of all employees who do not have supervisors.
Q14: SELECT FROM WHERE FNAME, LNAME EMPLOYEE SUPERSSN IS NULL
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Joined Relations Feature
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, etc)
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Joined Relations Feature
Examples: Q1:SELECT FROM WHERE FNAME, LNAME, ADDRESS EMPLOYEE, DEPARTMENT DNAME='Research' AND DNUMBER=DNO could be written as: Q1:SELECT FNAME, LNAME, ADDRESS FROM (EMPLOYEE JOIN DEPARTMENT WHERE ON DNUMBER=DNO) DNAME='Research ’
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Joined Relations Feature (contd.)
Another Example: Q2 could be written as follows; this illustrates multiple joins in the joined tables Q2: SELECT FROM WHERE PNUMBER, DNUM, LNAME, BDATE, ADDRESS ((PROJECT JOIN DEPARTMENT ON DNUM=DNUMBER) JOIN EMPLOYEE ON MGRSSN=SSN) PLOCATION='Stafford ’
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AGGREGATE FUNCTIONS
Include
COUNT, SUM, MAX, MIN, and AVG
Used to summarize information from multiple tuples into a single tuple summary Functions can be used in the SELECT clause or in a HAVING clause NULL values are discarded when aggregate functions are applied to a particular column Query 15: Find the maximum salary, the minimum salary, and the average salary among all employees.
Query 16: Find the maximum salary, the minimum salary, and the average salary among employees who work for the 'Research' department.
Q16: SELECT FROM WHERE MAX(SALARY), MIN(SALARY), AVG(SALARY) EMPLOYEE, DEPARTMENT DNO=DNUMBER AND DNAME='Research'
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AGGREGATE FUNCTIONS (contd.)
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: Q18: SELECT FROM SELECT FROM WHERE COUNT (*) EMPLOYEE COUNT (*) EMPLOYEE, DEPARTMENT 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
Partition
relation into subsets of tuples Create subgroups of tuples before summarizing 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
If NULLs exist in grouping attribute Separate group is created for all tuples with a NULL value in grouping attribute
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GROUPING (contd.)
Query 20: For each department, retrieve the department number, the number of employees in the department, and their average salary.
Q20: SELECT FROM GROUP BY DNO , COUNT (*), AVG (SALARY) EMPLOYEE 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
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GROUPING (contd.)
Query 21: For each project, retrieve the project number, project name, and the number of employees who work on that project.
Q21: SELECT FROM WHERE GROUP BY PNUMBER, PNAME, COUNT (*) PROJECT, WORKS_ON PNUMBER=PNO PNUMBER In this case, the grouping and functions are applied after the joining of the two relations