sql: interactive queries (2) prof. weining zhang

SQL: Interactive Queries (2)

Prof. Weining Zhang

Lecture 12 SQL: Interactive Queries (2) 2

Aggregate Functions Functions that take a set of tuples and

compute an aggregated value. Five standard functions: count, min, max, avg, sum They ignore null values. Find the total number, the average,

minimum, and maximum GPA of students whose age is 17.

select count(*), avg(GPA), min(GPA), max(GPA)

from Students where Age = 17


Aggregate Functions (cont.) Find id and name of students who take 5 or

more courses. select SID, Name from Students s where 5 <= (select count(distinct Cno) from Enrollment where SID = s.SID) Count(distinct Cno) distinct count(Cno).

Why? Must make sure the subquery generates a

value comparable in the predicate.


Group By Clause List id and name of students together with

the number of hours still needed to graduate, assuming 120 hours are required.

select s.SID, Name, 120 - sum(Hours) Hours-Needed from Students s, Enrollment e, Courses c where s.SID = e.SID and e.Cno = c.Cno and Grade <= ‘C’ group by s.SID, Name Enrolled courses are grouped by students.


Group By Clause (cont.) Aggregate functions often applied to

groups. One tuple is generated per group When using group by, select clause can

contain only grouping attributes and aggregate func.

Every grouping attribute must be in the select clause.The following is an illegal query (why?):

select Age, SID, avg(GPA) from Students group by Age


Having Clause For each student age group with more than

50 members, list the age and the number of students with that age.

select Age, count(*) from Students group by Age having count(*) > 50 Conditions on aggregate functions are

specified in the having clause. Select & Having may have different

functions.


Order By Clause List student names in ascending order. select Name from Students order by Name asc The default is ascending order. List students with GPA higher than 3.5, first

in descending order of GPA, and then in ascending order of name.

select * from Students where GPA > 3.5 order by GPA desc, Name asc


Some Complex Queries Find the average number of CS courses a

student takes. For non-CS major students who take more

CS courses than he does with his major courses, and have taken at lease 2 CS courses, list their id, name, number of CS courses, number of major courses, sorted first in descending order of number of CS courses, then in ascending order of name.


Interactive SQL Summary A query may have six clauses: select,

from, where, group by, having, order by. Conceptual evaluation of the query:

1. Evaluate From (cross product)2. Evaluate Where (selection)3. Evaluate Group By (form groups)4. Evaluate Aggregate functions on groups5. Evaluate Having (choose groups to

output)6. Evaluate Order By (sorting)7. Evaluate remaining Select (projection)


Interactive SQL Summary (count.) Many ways to express a query.

Flat queries may be more efficient. Nested queries may be easier to

understand. Duplicate elimination may be costly. <> (not equal) at predicate level often

gives a wrong answer. Use set difference, not in, not exists, etc. instead.

Need to handle null values explicitly. DBMSs often provide many convenient

functions. But need to check the compatibility.


Expressive Power of SQL SQL is relational complete.

Can express any relational algebraic query.

SQL is more powerful then relational algebra. Can express aggregation, ordering,

recursion, etc. SQL is not computational complete.

Can not do everything a general programming language can do.


Create Table Re-visited Can combine table creation with insertion of

tuples using a query.

create table Full-Professors as select FID, Name, Office from Faculty where Rank = ‘Full Professor’


Update By Queries Relation: Top_Students (SID, Name, GPA) Insert students with a GPA 3.8 or higher into

the Top_Students table. insert into Top_Students select SSN, Name, GPA from Students where GPA >= 3.8 Delete all students who take no courses. delete from Students where SID not in (select SID from Enrollment)


Update Statement For every student who takes Database I, set

the Grade to ‘A’. update Enrollment set Grade = 'A' where Cno in (select Cno from Courses where Title = ‘Database I')


Truncate vs Delete * Use delete to remove data and keep the

table storage space. delete from Departments; Use truncate to remove data and release

table storage space. truncate table Departments;


Views A view is a virtual table (as opposed to

stored base table) defined by a query, directly or indirectly, on base tables.

create view Top_Students as select SSN, Name, GPA from Students where GPA >= 3.8

A view may be defined in terms of other views.


Views (cont.) The query in view definition is usually not

executed until the view is queried. Typically, no data is stored for a view.

A view is queried as if it is a base table. Find name and GPA of top students whose

name starts with a `K'. select Name, GPA from Top_Students where Name like 'K%'


Query Modification Queries on a view are translated into

queries on base tables by folding the view. Previous query is translated first into: select Name, GPA from (select SSN, Name, GPA from Students where GPA >= 3.8) where Name like 'K%‘ Then into select Name, GPA from Students where GPA >= 3.8 and Name like 'K%'


Why Use Views? Data independence: keep existing

application programs from changes of base table schemas.

Access control: provide a mechanism for hiding sensitive data from certain users.

Productivity improvement: make user queries easier to express.


Example of Using ViewsConsider following base tables and a view: Students (SID, Name, Birthday, GPA, Phone) Emrollment(SID, Cno, Grade) Courses(Cno, Title, Hours, Dept) create view Student-Course as select SID, Name, Age(Birthday) Age, GPA, c.Cno, Title from Students s, Enrollment e, Courses c where s.SID=e.SID and e.Cno = c.Cno


Example of Using Views (cont.) Data independence: Applications using the

view are not affected if Age is stored or derived.

Access control: Phone and Birthday of students are hidden from users.

Productivity improvement: “Find all courses taken by a given student” is much simpler:

select Cno, Title from Student_Course where SID = X


Views and Updates What should happen if a user changes the

data in the Student-Course view? insert into Student-Course values (1234, ‘Dave Hall’, 32, 3.15,

‘CS334’, ‘B’) A view can not be updated if

Contains group by and aggregate functions

Involves multiple tables A single-table view can be updated if it

contains a key of the table


View Update Example * Which student should be deleted? create view Age_distribution as select Age, count(*) TotalNo from Students group by Age update Age_distribution set TotalNo = TotalNo – 1 where Age = 20 Which base relation should be changed? delete from Student_Course where SID = '1234'


Maintaining Materialized Views One may want to materialize a view (i.e.,

run its definition query and store the result) as is commonly done in industry (data warehouse). (Why?)

View Maintenance: How to maintain the consistency between a view and its base tables, when base tables are updated?

Incremental View Maintenance: How to maintain a view without re-computing the entire view?


Assertions Constraints defined over multiple tables. No student is allowed to take more than six

courses.SQL> create assertion Course_Constraint check (not exists (select * from Students s where 6 < (select count(*) from Enrollment where SID = s.SID)));


RecursionExamples of Recursive Queries. Ancestors

Relation: ParentOf(Parent, Child) Query: Find all of Mary's ancestors

Company hierarchy Relations: Employee(ID, Salary)

Manager(MgrID, EmpID) Project(Name, MgrID)

Query: What's the total salary cost of project "X“


With Statement with R1 as (query),

..., Rn as (query)<query involving R1, ..., Rn & other relations>

Conceptual Evaluation Compute R1, ..., Rn into temporary

relations Evaluate the query Destroy R1, ..., Rn

Can also specify schema for Ri's:with R1(A1, A2, ..., Am) as (query), ...


Example of With Relation: Apply(ID, Name, Location, Date) with DL as(select ID, Date from Apply where Location = ‘Dallas’),

HO as (select ID, Date from Apply where Location = ‘Houston’)

select ID, DL.Date DLdate, HO.Date HOdate from DL, HO where DL.ID = HO.ID

Just like "temporary view definitions". The Ri's can be recursive or mutually

recursive- Must use keyword recursive- Usually need to union base case & recursion


Recursion in SQL Find Mary's ancestors from ParentOf relation.

with recursive Ancestor(Anc, Desc) as ((select Parent Anc, Child Desc from ParentOf) union (select A.Anc Anc, P.Child Desc from Ancestor A, ParentOf P where A.Desc = P.Parent))select Anc from Ancestor where Desc = ‘Mary’

Ancestor = ParentOf Repeat Ancestor = Ancestor joins ParentOf Until no more changes to Ancestor


Restrictions & Features Only support “linear recursion”: each from-

clause can have at most one recursively defined relation.

With relations can be defined as views. Not evaluated until being queried (Why useful?).

Can define “mutual recursion”: two recursive relations mutually define each other.


Commit and Rollback Changes to data in a user session may not

be visible to other users immediately (why?) Use commit to make changes made by

insert, delete and update permanent and visible to other users.

Use rollback to undo uncommitted changes made by insert, delete and update.

Normal exit performs a commit. Abnormal exit performs a rollback. Used for transaction processing (more

later).


Grant Statement The owner of a table can grant privileges of

access to the table to other users. Syntax: grant {all | privilege {, privilege}

on table_name | view_name to {public | user_name {, user_name} } [with grant option] Privileges: select | delete | insert | update [column_name {,

column_name ...}] | references [column_name {,

column_name ...}]


Sample Grant Statements Grant select and insert access to Students

table to users john and terry. grant select, insert on Students to john,

terry Grant all privileges to user john. grant all on Students to john Allow all user to update Age and GPA. grant update (Age, GPA) on Students to

public Allow user john to create a foreign key to

referencing SID. grant references (SID) on Students to john


Features of Grant The owner of a table has all privileges. Public includes current and future users. If columns are not named, all current and

future columns are implied. To grant privileges on a view, one must be

the owner of the view and have the privileges on all base tables used to define the view.

With grant option allows the grantee to grant the privileges transitively.


Roles A role is a named group of privileges that

can be granted to users. Used to ease the task of granting privileges.

create role TA; grant create table, create view to TA; grant TA to Wang, Johnson;


Create External Schema * Create a view and grant privileges to a

group of intended users. Allow John the select and insert access only

to SID, Name and Age of students with GPA higher than 3.8.

create view Stud1 as select SID, Name, Age from Students where GPA > 3.8 grant select, insert on Stud1 to john


Revoke Statement Revoke granted privileges on DB objects. Syntax: revoke {all | privilege {, privilege ...} } on table_name | view_name from {public | user_name {,

user_name ...}} Owner's privileges can not be revoked. Revoke all privileges of John on Students. revoke all on Students from john


Sequence in Oracle SQL An Oracle object for generating a sequence

of integer values. Often used to generate unique key values.

Syntax of create sequence: create sequence sequence_name [increment by integer] [start with integer] [maxvalue integer | nomaxvalue] [minvalue integer | nominvalue] [cycle | nocycle]


Sample Sequences create sequence emp_seq start with 1000; create sequence even_seq increment by 2 start with 2 maxvalue

2000; create sequence negative_seq increment by -1 start with -1; Default increment value is 1. Default start value for positive (negative)

increment value is minvalue = 1 (maxvalue).

With cycle specified, the integers between minvalue and maxvalue will be recycled


Use Sequences Two functions for sequence seq:

seq.currval returns the current value of seq.

seq.nextval returns the next value of seq.

insert into Employees(Emp_no, Name,Age)

values (emp_seq.nextval, ‘John’, 22);

nextval must be used at least once before using currval.


Oracle SQL*Loader sqlldr (SQL*Loader) is a Unix command (in

CSLab) to load data into an Oracle table from a Unix text file.

Requires two files: control file (with extension .ctl) data file (with extension .dat)

Usage: sqlldr userid/passwd control=foo.ctl Other options: direct (direct load), skip

(skip n lines), load (load m lines) Will generate .log .bad files.


A Sample Control FileThe control file is pub.ctl for Publishers table.

load data infile 'pub.dat’ into table publishers fields terminated by "," (pub_id, pub_name, city, state)

It expects an empty table. Other options: append into, replace into.


A Sample Data FileThe data file is pub.dat. 0736,New Age Books,Boston,MA 0877,Binnet & Hardley,Washington,DC 1111,stone Age BooAo,Boston,MA 1389,Algodata Infosystems,Berkeley,CA 2222,Harley % adkfj,Wash,DC 3333,adfadh adfhj,Berkey,CA

Other formats of control and data files are also supported.


Look Ahead Next topic: More advanced features of SQL Read textbook:

Oracle8 book: Chapter 3.

sql: interactive queries (2) prof. weining zhang

Documents