SQL querys in detail || Sql query slides

SQL Query Slides
1By:-Gourav Kottawar

Retrieval Queries in SQL
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

Relational Database schema

Populated
Database:

Simple Queries
Query 0: Retrieve the birthdate and address of the employee whose
name is 'John B. Smith'.
Query 1: Retrieve the name and address of all employees who work for
the 'Research' department.
Q0: SELECT BDATE, ADDRESS
FROM EMPLOYEE
WHERE FNAME='John' AND MINIT='B’
AND LNAME='Smith’
Q1: SELECT FNAME, LNAME, ADDRESS
FROM EMPLOYEE, DEPARTMENT
WHERE DNAME='Research' AND
DNUMBER=DNO

Some Queries Cont.
Q2: SELECT PNUMBER, DNUM, LNAME, BDATE, ADDRESS
FROM PROJECT, DEPARTMENT, EMPLOYEE
WHERE DNUM=DNUMBER AND MGRSSN=SSN AND
PLOCATION='Stafford'
Query 3: For each employee, retrieve the employee's name, and the name
of his or her immediate supervisor.
Q3: SELECT E.FNAME, E.LNAME, S.FNAME, S.LNAME
FROM EMPLOYEE E S
WHERE E.SUPERSSN=S.SSN
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.

Some Queries Cont.
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')
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.

Some Queries Cont.
Q5: 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)
Query 5: Retrieve the name of each employee who has a dependent
with the same first name as the employee.
Q5A: SELECT E.FNAME, E.LNAME
FROM EMPLOYEE E, DEPENDENT D
WHERE E.SSN=D.ESSN AND
E.FNAME=D.DEPENDENT_NAME
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

Some Queries Cont. EXISTS
Q5B: SELECT FNAME, LNAME
FROM EMPLOYEE
WHERE EXISTS (SELECT *
FROM DEPENDENT
WHERE SSN=ESSN AND
FNAME=DEPENDENT_NAME)
EXISTS is used to check whether the result of a correlated nested query
is empty (contains no tuples) or not

Some Queries Cont.
explicit (enumerated) set of values
Query 6: Retrieve the social security numbers of all employees who work
on project number 1, 2, or 3.
Q6: SELECT DISTINCT ESSN
FROM WORKS_ON
WHERE PNO IN (1, 2, 3)
It is also possible to use an explicit (enumerated) set of values in the
WHERE-clause rather than a nested query

Some Queries Cont.
Query 7: Retrieve the name of each employee who works on all the projects
controlled by department number 5.
Q7: SELECT FNAME, LNAME
FROM EMPLOYEE
WHERE ( (SELECT PNO
FROM WORKS_ON
WHERE SSN=ESSN)
CONTAINS
(SELECT PNUMBER
FROM PROJECT
WHERE DNUM=5) )
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).

Some Queries Cont. Null Value
Query 8: Retrieve the names of all employees who do not have supervisors.
Q8: SELECT FNAME, LNAME
FROM EMPLOYEE
WHERE SUPERSSN IS NULL
SQL uses IS or IS NOT to compare NULLs because it considers each NULL
value distinct from other NULL
Note: If a join condition is specified, tuples with NULL values for the join
attributes are not included in the result

Some Queries Cont. JOIN
Can be written as:
QTA: SELECT E.FNAME, E.LNAME, S.FNAME, S.LNAME
FROM (EMPLOYEE E LEFT OUTER JOIN EMPLOYEES
ON E.SUPERSSN=S.SSN)
QT: SELECT E.FNAME, E.LNAME, S.FNAME, S.LNAME
FROM EMPLOYEE E S
WHERE E.SUPERSSN=S.SSN

Some Queries Cont. JOIN
Can be written as:
Q9A: SELECT FNAME, LNAME, ADDRESS
FROM (EMPLOYEE JOIN DEPARTMENT
ON DNUMBER=DNO)
WHERE DNAME='Research’
Q9: SELECT FNAME, LNAME, ADDRESS
WHERE DNAME='Research' AND DNUMBER=DNO
Or as:
Q9B: SELECT FNAME, LNAME, ADDRESS
FROM (EMPLOYEE NATURAL JOIN
DEPARTMENT AS DEPT(DNAME, DNO, MSSN, MSDATE)
WHERE DNAME='Research’

Joined Relations Feature
in SQL2
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 B: SELECT PNUMBER, DNUM,
LNAME, BDATE, ADDRESS
FROM (PROJECT JOIN
DEPARTMENT ON
DNUM=DNUMBER) JOIN
EMPLOYEE ON
MGRSSN=SSN) )
WHERE PLOCATION='Stafford’

AGGREGATE FUNCTIONS
Query 10: Find the maximum salary, the minimum salary, and
the average salary among all employees.
Q10: SELECT MAX(SALARY), MIN(SALARY), AVG(SALARY)
FROM EMPLOYEE
Include COUNT, SUM, MAX, MIN, and AVG
Query 11: Find the maximum salary, the minimum salary, and the average
salary among employees who work for the 'Research' department.
Q11: SELECT MAX(SALARY), MIN(SALARY), AVG(SALARY)
WHERE DNO=DNUMBER AND
DNAME='Research'

Group by
Query 12: For each department, retrieve the department number, the
number of employees in the department, and their average salary.
Q12: SELECT DNO, COUNT (*), AVG (SALARY)
FROM EMPLOYEE
GROUP BY DNO
SQL has a GROUP BY-clause for specifying the grouping attributes, which
must also appear in the SELECT-clause
Query 13: For each project, retrieve the project number, project
name, and the number of employees who work on that project.
Q13: SELECT PNUMBER, PNAME, COUNT (*)
FROM PROJECT, WORKS_ON
WHERE PNUMBER=PNO
GROUP BY PNUMBER, PNAME

Group by cont. Having
Query 14: 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.
Q14: SELECT PNUMBER, PNAME, COUNT (*)
FROM PROJECT, WORKS_ON
WHERE PNUMBER=PNO
GROUP BY PNUMBER, PNAME
HAVING COUNT (*) > 2
lThe HAVING-clause is used for specifying a selection condition on groups
(rather than on individual tuples)

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>]

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

More complex Select “SQL Server”
SELECT [ ALL | DISTINCT ]
[ TOP n [ PERCENT ] [ WITH TIES ] ]
< select_list >
< select_list > ::=
{ *
| { table_name | view_name | table_alias }.*
| { column_name | expression | IDENTITYCOL |
ROWGUIDCOL }
[ [ AS ] column_alias ]
| column_alias = expression
} [ ,...n ]
SELECT select_list
[ INTO new_table ]
FROM table_source
[ WHERE search_condition ]
[ GROUP BY group_by_expression ]
[ HAVING search_condition ]
[ ORDER BY order_expression [ ASC |
DESC ] ]
Select Clause:
From Clause:
[ FROM { < table_source > } [ ,...n ] ]
< table_source > ::=
table_name [ [ AS ] table_alias ] [ WITH ( < table_hint > [
,...n ] ) ]
| view_name [ [ AS ] table_alias ]
| rowset_function [ [ AS ] table_alias ]
| OPENXML
| derived_table [ AS ] table_alias [ ( column_alias [ ,...n ] ) ]
| < joined_table >
< joined_table > ::=
< table_source > < join_type > < table_source > ON <
search_condition >
| < table_source > CROSS JOIN < table_source >
| < joined_table >
< join_type > ::=
[ INNER | { { LEFT | RIGHT | FULL } [ OUTER ] } ]
[ < join_hint > ]
JOIN
Arguments
< table_source >

More complex Select “SQL Server”
Cont.
Where Clause:
[ WHERE < search_condition > | <
old_outer_join > ]
< old_outer_join > ::=
column_name { * = | = * } column_name
Group by clause:
[ GROUP BY [ ALL ] group_by_expression
[ ,...n ]
[ WITH { CUBE | ROLLUP } ]
]
Having:
[ HAVING < search_condition > ]
Order By Clause:
[ ORDER BY { order_by_expression [ ASC |
DESC ] } [ ,...n] ]
Compute Clause:
[ COMPUTE
{ { AVG | COUNT | MAX | MIN | STDEV | STDEVP
| VAR | VARP | SUM }
( expression ) } [ ,...n ]
[ BY expression [ ,...n ] ]
]

Compute
Row aggregate
function Result
AVG Average of the values in the numeric expression
COUNT Number of selected rows
MAX Highest value in the expression
MIN Lowest value in the expression
STDEV Statistical standard deviation for all values in the expression
STDEVP |Statistical standard deviation for the population for all values in the expression
SUM Total of the values in the numeric expression
VAR Statistical variance for all values in the expression
VARP Statistical variance for the population for all values in the expression

SQL querys in detail || Sql query slides

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SQL querys in detail || Sql query slides