第3章关系数据库SQL语言idke.ruc.edu.cn/xfmeng/course/Introduction to Database... · 2014-03-17 · 数据库系统概论讲义，第三章SQL语言，2014，2. Computation

数据库系统概论讲义，第三章 SQL语言，2014，2

第3章关系数据库SQL语言

孟小峰

[email protected]

信息学院

2014/3/13

http://www.cs.wisc.edu/~dbbook/images/book3ed.jpg�

http://www.pearson.ch/bigcovers/0321210255.aspx�


上节课……

• SQL：– SQL86，SQL89，SQL92，SQL99– DDL，DML，DCL

• DDL– 基本表，索引，视图

– CREATE TABLE，CREATE INDEX– ALTER TABLE, – DROP TABLE, DROP INDEX– 数据字典与DDL的处理

• Basic Select


Computation in SQL (1)Consider

Employees (SSN, Name, Monthly_Salary)

Example: Find the SSN, name and annual salary of each employee.

select SSN, Name, 12*Monthly_Salary from Employees

Result: SSN Name 12*Monthly_Salary------ ------- -------------------------… …… ……


Computation in SQL (2)• Naming the expression in the output.

– In Oracle, 12*Monthly_Salary will be used for our example.

– A more general solution is to assign a new name to the column.

select SSN, Name, 12*Monthly_Salary as Annual_Salary

from Employees• “as” is optional.

Result: SSN Name Annual_Salary------ ------- -------------------------… …… ……


Computation in SQL (3)

例使用列别名改变查询结果的列标题SELECT Sname NAME， 'Year of Birth: ' BIRTH，2014-Sage

BIRTHDAYFROM Student；输出结果：

NAME BIRTH BIRTHDAY

------- ---------------- ---------李勇 Year of Birth: 1976刘晨 Year of Birth: 1977王名 Year of Birth: 1978张立 Year of Birth: 1978



Some remarks:• The set of mathematical operators

supported includes +, -, *, /.

• Other operators include length(x), lower(x), upper(x), x || y, …

• Computations and functions can be used in select and where clauses.



Examples:

select SSN, upper(First_Name || ' '

|| Last_Name)from Employees

select substr(SSN, 6, 4), Gradefrom Enrollmentwhere Course_no = 'CS532'



Common Oracle Functions:mod(m,n) : remainder of m divided by n power(x,y) : x raised to the power yround(n,m) : round n to the m-th digit

following the point


Computation in SQL (6)Common Oracle Functions:sign(x) : 0 if x = 0; 1 if x > 0; -1 if x < 0 sqrt(x) : the square root of xinitcap(s) : change the first char of each word in s

to uppercaselower(s) : change all chars in s to lowercasereplace(s,s1,s2) : change each occurrence of s1 in s

to s2 substr(s,m,n) : select n chars from s starting at the

m-th char



Common Oracle Functions:length(s) : the length of s sysdate : the current dateuser: user id of the current user last_day : the last day of current month to_char(x) : convert x to char data type to_number(x) : convert numeric string x to

number type to_date(x) : convert x of proper format to date

type



to_char(x) is used for displaying date and time in different formats.

• Format: to_char(attribute, ‘format_mask’)Examples:• select name, to_char(birthdate,

‘MM/DD/YYYY’) birth_date from students;• Select name, to_char(birthdate,

‘MM/DD/YYYY day HH:MM:SS’) birth_time from students;



• Oracle’s decode function.select SSN, Name,

decode(Year, 1, ‘freshman’,2, ‘sophomore’,3, ‘junior’,4, ‘senior’) Status

from Students



• Any computation involves a null value yields a null value.

Eaxmple: Consider table Employees

Emp# Name Salary Bonus

123 Smith 25000 5000234 John 28000 null



select Name, Salary+Bonus Total_wagefrom Employees

result: Name Total_wage

Smith 30000John null



• Use NVL function to convert null valuesformat: nvl(exp1, exp2)– exp1 is a source value that may be null– exp2 is the target value for converting null

Ex: select Name, Salary + nvl(Bonus, 0) Total_wage

from Employeeswill compute all total wages correctly.



select Name, Salary+ nvl(Bonus, 0) Total_wage from Employees

result: Name Total_wage

Smith 30000John 2800


Find the names and GPAs of all students who take database systems.

select Name, GPAfrom Students, Enrollment, Courseswhere Title = `database systems'

and Students.SSN = Enrollment.SSNand Enrollment.Course_no =

Courses.Course_no

连接查询(1)


连接查询(2)

The semantics of the previous query can be expressed (almost correctly!) as:

π Name, GPA (σ Title = `database systems’ and Students.SSN =

Enrollment.SSN and Enrollment.Courses_no = Courses.Course_no

(Students × Enrollment × Courses))= π Name, GPA (Students (Enrollment

σ Title = `database systems'(Courses)))


连接查询(3)

In general,select distinct Ri.A, Rj.B, ..., Rk.Cfrom R1, R2, ..., Rn where Conditions

is equivalent toπ Ri.A, Rj.B, ..., Rk.C (σConditions

(R1 × R2 × ... × Rn))


连接查询(4)Example: Find the names and GPAs of all students who take

database systems.

select Name, GPA from Students s, Enrollment e, Courses cwhere Title = `database systems'

and s.SSN = e.SSN and e.Course_no = c.Course_no

RANGE OF c is CoursesRANGE OF s is StudentsRANGE OF e is Enrollment{(s.name，s.GPA) | ∃s (s.SSN=e.SSN∧e.Course_no=c.Course_no)}


连接查询(5)

• s, e and c are (relational) tuple variables (aliases, correlation names) for relations Students, Enrollment and Courses.

• Tuple variables can be used to simplify query specification and save time.

• Tuple variables are also useful when tuples from the same relation need to be compared.


连接查询(6)

Example: Find all pairs of students who have the same GPA.

select s1.SSN, s2.SSN from Students s1, Students s2where s1.GPA = s2.GPA

and s1.SSN < s2.SSN• Question: Why use “s1.SSN < s2.SSN”?


连接查询(7)Example: Find the names of all students whose

GPA is higher than Tom's GPA.select s1.Name from Students s1, Students s2where s2.Name = `Tom'

and s1.GPA > s2.GPA • Question: Is a student qualified if his/her GPA

is higher than some Tom's GPAs but not all Tom's GPAs?


连接查询(8)

s1 SSN Name GPA123456789 John 3.6234567891 Tom 3.2345678912 Tom 3.8

s2 SSN Name GPA123456789 John 3.6234567891 Tom 3.2345678912 Tom 3.8

• Joins imply the existence semantics.


Joining Tables in From Clause (1)Example: Find the titles of all courses offered by

departments located in building INFO.Departments (Name, Location, Chairman)Departments1 (Dept_Name, Location,

Chairman)select Courses.Titlefrom Courses join Departments on

Dept_Name = Namewhere Location = ‘INFO'


Joining Tables in From Clause (2)or

select Courses.Titlefrom Courses join Departments1 using

(Dept_Name)where Location = 'EB'

orselect Courses.Titlefrom Courses natural join Departments1where Location = 'EB'


Outerjoin (1)R1 R2 R1 R2

A B C C D E A B C D Ea1 b1 c1 c1 d1 e1 a1 b1 c1 d1 e1 a4 b3 c2 c6 d3 e2

• The second tuples of R1 and R2 are not present in the result (called dangling tuples).

• Applications exist that require to retain dangling tuples.


Outerjoin (2)

10. o --- outer joinFormat: R1 o R2 Semantics: like join except (a) it retains dangling

tuples from both R1 and R2; (b) it uses null to fill out missing entries.

R1 o R2

A B C D E a1 b1 c1 d1 e1 a4 b3 c2 null nullnull null c6 d3 e2


Left Outerjoin and Right Outerjoin

11. --- left outer join Format: R1 R2 Semantics: like outerjoin but retains only

dangling tuples of the relation on the left.

12. --- right outer join Format: R1 R2 Semantics: like outerjoin but retains only dangling tuples of the relation on the right.


Outerjoining Tables in From Clause (1)

Employees (SSN, Name, Position, Proj_no)Projects(Proj_no, Title, Budget)Example: Find who works for which project.

• Not quite correct:

select SSN, Name, Title from Employees e, Projects pwhere e.Proj_no = p.Proj_no


Outerjoining Tables in From Clause (2)

• Identify also those who do not work for any project.

select SSN, Name, Title

from Employees left join Projects onEmployees.Proj_no = Projects.Proj_no


Outerjoining Tables in From Clause (3)• Identify also those projects no one works for.

select SSN, Name, Titlefrom Employees right join Projects

using (Proj_no)• Identify even those who do not work for any

project and those projects no one works for:select SSN, Name, Title from Employees natural full join

Projects


集合查询 (1)

• SQL supports three set operations: union, intersect, except (Sybase) or minus (Oracle version of set difference)

• Union compatibility is required.

Consider the following two relations: G_Students(SSN, Name, GPA, GRE, Dept_Name)

Instructors(SSN, Name, Office, Salary, Dept_Name)


集合查询 (2)

Example: Find the names of those people who are either a graduate student or an instructor or both in CS department.(select Name from G_Students

where Dept_Name = 'CS')union(select Name from Instructorswhere Dept_Name = 'CS')

• union removes duplicate rows.• union all keeps duplicate rows.


集合查询 (3)

Example: Find the SSNs and names of those who are both a graduate student and an instructor in CS department.

(select SSN, Name from G_Studentswhere Dept_Name = 'CS')

intersect(select SSN, Name from Instructors where Dept_Name = 'CS')


集合查询 (4)

Example: Find the SSNs and names of those who are an instructor but not a graduate student in CS department.

(select SSN, Name from Instructors where Dept_Name = 'CS')minus(select SSN, Name from G_Studentswhere Dept_Name = 'CS')


集合查询 (4)

Example: Find all students who are 20, 22, or 24 years old.

select * from Studentswhere Age in (20, 22, 24)

• not in is the opposite of in.


嵌套查询(1)

Example: Find the names of all students who take at least one course offered by the CS department.select Name from Students s, Enrollment ewhere s.SSN = e.SSN and e.Course_no in

(select Course_no from Courses where Dept_Name = 'CS')


嵌套查询(2)

• The previous query is a nested query. The query outside is an outer query and the query nested under the outer query is an inner query. Multiple nestings are allowed.

• The semantics of the above query is to evaluate the inner query first and evaluate the outer query last.

• Many nested queries have equivalent non-nested versions.


嵌套查询(3)The previous query is equivalent to:(1) select Name

from Students s, Enrollment e, Courses cwhere s.SSN = e.SSN and e.Course_no =

c.Course_no and c.Dept_Name = 'CS’

(2) select Name from Students where SSN in(select SSN from Enrollment

where Course_no in(select Course_no from Courses where Dept_Name = 'CS'))


• 第三章习题1-4• 补充习题

dbhw2中的查询练习


Book Time

• 阅读：

– 《民国风度》，徐百柯

– 曾经有那样一个时代，曾经有那样一批人物。他们那相样地想着，那样地活着。他们离我们今天并不遥远，但他们守护、在意、体现的精神、传统、风骨，已与我们相去甚远

– 冯友兰：两束雄文，一抹背影

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第3章关系数据库SQL语言idke.ruc.edu.cn/xfmeng/course/Introduction to Database... · 2014-03-17 · 数据库系统概论讲义，第三章SQL语言，2014，2. Computation