Chapter 3 | Introduction to SQL¶

约 5358 个字 294 行代码预计阅读时间 30 分钟

Data Definition Language¶

The SQL data-definition language (DDL) allows the specification of information about relations, including:

The schema for each relation.
The domain of values associated with each attribute.
Integrity constraints
The set of indices to be maintained for each relations.
Security and authorization information for each relation.
The physical storage structure of each relation on disk.

Domain Types in SQL¶

char(n) : Fixed length character string, with user-specified length n.
- 具有用户指定长度n的固定长度的字符串。也可以使用全称形式character
varchar(n) : Variable length character strings, with user-specified maximum length n.
- 具有用户指定的最大长度n的可变长度的字符串。等价的全称形式是character varying
int : Integer (a finite subset of the integers that is machinedependent).
smallint : Small integer (a machine-dependent subset of the integer domain type).
numeric(p,d) : Fixed point number, with user-specified precision of p digits, with d digits to the right of decimal point.
- 具有用户指定精度的定点数。这个数有 p 位数字(加上一个符号位).并且小数点右边有 p 位中的 d 位数字。
real, double precision : Floating point and double-precision floating point numbers, with machine-dependent precision.
float(n) : Floating point number, with user-specified precision of at least n digits.
- 精度至少为n位数字的浮点数。

Note

定长字符串，在 c 语言中字符串结尾有 \0，但在 SQL 中不是，你说是多长就读到多长。

整数长度与机器有关。

p 是有效数字是几位，d 是小数点后几位。number(3,1) allows 44.5 to be store exactly, but neither 444.5 or 0.32

Built-in Data Types in SQL

date : Dates, containing a (4 digit) year, month and date. 日历日期，包括年(四位)、月和月中的日。
time : Time of day, in hours, minutes and seconds. 一天中的时间，用时、分和秒来表示。可以用变量 time(p) 来指定秒的小数点后的数字位数(缺省值为0)。通过指定 time with timezone，还可以把时区信息连同时间一起存储。
timestamp : date plus time of day. date 和 time 的结合。可以用变量 timestamp(p) 来指定秒的小数点后的数字位数(缺省值为6)。如果指定 with timezone，则时区信息也会被存储。
interval : period of time

Interval values can be added to date/time/timestamp values date, time functions:

current_date(), current_time()

year(x), month(x), day(x), hour(x), minute(x), second(x)

可以从 date/time/timestamp 中提取出年月日时分秒

Example

date '2005-7-27'

time '09:00:30'

timestamp '2005-7-27 09:00:30.75'

interval '1' day

Subtracting a date/time/timestamp value from another gives an interval value.

我们可以利用 extract(field from d) 来从 date 或 time 值 d 中提取出单独的域，这里的域(field)可以是 year 、 month 、 day 、hour、minute 或 second 中的一种。时区信息可以用 timezone hour 和 timezone minute 来提取。

SQL 定义了一些函数来获取当前的日期和时间。例如:

current date 返回当前日期
current time 返回当前时间(带有时区)，
localtime 返回当前的本地时间(不带时区)
current timestamp (带有时区)以及 localtimestamp (本地日期加时间，不带时区)返回时间戳（日期加上时间）。

Type Conversion and Formatting Functions¶

虽然系统会自动执行某些数据类型的转换( conversion )，但其他的转换需要显式请求。

我们可以使用形如 cast(e as t) 的表达式来将表达式 e 转换为类型 t 。可能需要数据类型转换来执行特定的操作或强制保证特定的排序次序。

例如，请考虑 instructor 的 ID 属性，我们已将其指定为字符串(varchar(5))。如果我们按此属性排序输出，则 ID '11111' 位于 ID '9' 之前，因为第一个字符 '1' 在 '9' 之前。但是，如果我们写:

select cast(ID as numeric(5))as inst_id
from instructor
order by inst_id

其结果将会是我们想要的排序次序。

为查询结果显示的数据可能需要不同类型的转换。

例如，我们可能希望数值以特定位数的数字来显示，或者数据以特定格式(例如月-日-年或日-月-年)来显示。

显示格式的这些变化并不是数据类型的转换，而是格式的转换。

数据库系统提供了各种格式化函数且相关细节因主流系统而异。

MySQL提供了 format 函数。
Oracle和PostgreSQL提供了组函数: to char、to number 和 to date。
SQL Server 提供了 convert 函数。

结果显示中的另一个问题是处理空值。在本书中，我们使用 null 来使阅读更清晰，但在大多数系统中缺省设置只是将字段留空。我们可以使用 coalesce 函数来选择在查询结果中输出空值的方式。

该函数接收任意数量的参数(所有参数必须是相同的类型)，并返回第一个非空参数。

例如，如果我们希望显示教师的 ID 和工资，但是将空工资显示为0，我们会写:

select ID,coalesce(salary,0)as salary 
from instructor

coalesce 的一个限制是要求所有参数必须是相同的类型。如果我们希望将空工资显示为 'N/A' 以表示"不可用"，我们将无法使用 coalesce。

Default Values¶

SQL 允许为属性指定缺省(default)值，如下面的 create table 语句所示:

create table student
(ID       varchar (5),
name      varchar (20) not null,
dept_name varchar(20),
tot_cred  numeric (3,0) default 0,
primary key (ID));

tot_cred 属性的缺省值被声明为0。其结果是，当一个元组被插入 student 关系中时，如果没有给出 tot_cred 属性的值，那么该元组在此属性上的取值就被置为0。

Create Table Construct¶

An SQL relation is defined using the create table command :

create table $r(A_1 \ D_1, A_2 \ D_2, \cdots , A_n \ D_n,(integrity_{constraint_1}),\cdots,(integrity_{constraint_k}))$

r is the name of the relation
each $A_i$ is an attribute name in the schema of relation r
$D_i$ is the data type of values in the domain of attribute $A_i$

Example

create table instructor(
    ID char(5),
    name varchar(20) not null,
    dept_name varchar(20),
    salary numeric(8,2))
insert into instructor values('10211', 'Smith', 'Biology', 66000); /* legal */
insert into instructor values ('10211', null, 'Biology', 66000); /* illegal */

Integrity Constraints in Create Table¶

not null
primary key $(A_1, \cdots, A_n)$
foreign key $(A_m, \cdots, A_n) \ references \ r$

Declare ID as the primary key for instructor

create table instructor (
    ID char(5),
    name varchar(20) not null,
    dept_name varchar(20),
    salary numeric(8,2),
    primary key (ID),
    foreign key (dept_name) references department)

primary key declaration on an attribute automatically ensures not null

And a Few More Relation Definitions¶

create table student (
    ID varchar(5),
    name varchar(20) not null,
    dept_name varchar(20),
    tot_cred numeric(3,0) default 0，
    primary key (ID),
    foreign key (dept_name) references department);

create table takes (
    ID varchar(5),
    course_id varchar(8),
    sec_id varchar(8),
    semester varchar(6),
    year numeric(4,0),
    grade varchar(2),
    primary key (ID, course_id, sec_id, semester, year),
    foreign key (ID) references student,
    foreign key (course_id, sec_id, semester, year) references section );

Note

DEFAULT 关键字用于指定列的默认值。当插入新记录时，如果没有为该列提供值，数据库会自动使用默认值。
sec_id can be dropped from primary key above, to ensure a student cannot be registered for two sections of the same course in the same semester.

And more still¶

create table course (
    course_id varchar(8) primary key,
    title varchar(50),
    dept_name varchar(20),
    credits numeric(2,0),
    foreign key (dept_name) references department (dept_name));
    foreign key (dept_name) references department
    on delete cascade |set null |restrict |set default
    on update cascade |set null |restrict |set default

被引用掉的元组，如果删掉了，级联（多米诺连锁反应）删除，或者设置为null，或者拒绝删除，或者设置为默认值。

Drop and Alter Table Constructs¶

drop table student : Deletes the table and its contents.
delete from student : Deletes all contents of table, but retains table.
alter table :
- alter table r add A D :
  - where A is the name of the attribute to be added to relation r and D is the domain of A.
  - All tuples in the relation are assigned null as the value for the new attribute.
  - e.g. alter table student add resume varchar(256);
- alter table r drop A :
  - where A is the name of an attribute of relation r.
  - Dropping of attributes not supported by many databases.

SQL and Relational Algebra¶

e.g.

select A1, A2, ... An
from r1, r2, ..., rm
where P

is equivalent to the following expression in multiset relational algebra

\[\Pi_{A1, A2, ..., An}(r1 \times r2 \times ... \times rm)\]

select A1, A2, sum(A3)
from r1, r2, ..., rm
where P
group by A1, A2

is equivalent to the following expression in multiset relational algebra

\[_{A1,A2} \ G_{sum(A3)}(\sigma_P(r1 \times r2 \times ... \times rm))\]

Basic Query Structure¶

The SQL data-manipulation language (DML) provides the ability to query information, and insert, delete and update tuples.

A typical SQL query has the form:

select A1, A2, ..., An
from r1, r2, ..., rm
where P

A1 represents an attribute
r1 represents a relation
P is a predicate

Info

在 sql 中不区分大小写，这是历史遗留问题。

The result of an SQL query is a relation.

The `select` Clause¶

The select clause list the attributes desired in the result of a query, corresponds to the projection operation of the relational algebra.

Example: find the names of all instructors:

answer

select name
from instructor

SQL allows duplicates in relations as well as in query results.

To force the elimination of duplicates, insert the keyword distinct after select.

Example: Find the names of all departments with instructor, and remove duplicates

answer

select distinct dept_name
from instructor

The keyword all specifies that duplicates not be removed.

answer

select all dept_name
from instructor

An asterisk(*) in the select clause denotes "all attributes".

answer

select *
from instructor

The select clause can contain arithmetic expressions involving the operation, +, –, *, and /, and operating on constants or attributes of tuples.

Example: Find a relation that is the same as the instructor relation, except that the value of the attribute salary is divided by 12.

answer

select ID, name, dept_name, salary/12
from instructor

The `where` Clause¶

The where clause specifies conditions that the result must satisfy.

Corresponds to the selection predicate of the relational algebra.

Example: To find all instructors in Comp. Sci. dept with salary > 80000

answer

select name
from instructor
where dept_name = 'Comp. Sci.' and salary > 80000

Comparison results can be combined using the logical connectives and, or, and not. Comparisons can be applied to results of arithmetic expressions.

SQL includes a between comparison operator.

Example: Find the names of all instructors with salary between $\$90,000 \ and \ \$100,000 \ (that \ is, \ge \$90,000 \ and \ \le \ \$100,000)$

answer

select name
from instructor
where salary between 90000 and 100000

Example: Tuple comparison

SQL 允许我们用符号 ($v_1, \ldots, v_n$) 来表示一个包含值 $v_1, \ldots, v_n$ 的 n 维元组。该符号被称为行构造器(row constructor)。在元组上可以使用比较运算符，并按字典顺序进行比较运算。

answer

select name, course_id
from instructor, teaches
where (instructor.ID, dept_name) = (teaches.ID, 'Biology');

The `from` Clause¶

The from clause lists the relations involved in the query.

Corresponds to the Cartesian product operation of the relational algebra.

Example: Find the Cartesian product instructor $\times$ teaches.

answer

select *
from instructor, teaches

Cartesian product not very useful directly, but useful combined with where-clause condition (selection operation in relational algebra).

`Joins`¶

Example: For all instructors who have taught some course, find their names and the course ID of the courses they taught.

answer

select name, course_id
from instructor, teaches
where instructor.ID = teaches.ID

Example: Find the course ID, semester, year and title of each course offered by the Comp. Sci. department.

answer

select section.course_id, semester, year, title
from section, course
where section.course_id = course.course_id and dept_name = 'Comp. Sci.'

`Natural Join`¶

Natural join matches tuples with the same values for all common attributes, and retains only one copy of each common column.

Example:

select name, course_id
from instructor, teaches
where instructor.ID = teaches.ID;

select name, course_id
from instructor natural join teaches;

Attention

Beware of unrelated attributes with same name which get equated incorrectly

Example: List the names of instructors along with the titles of courses that they teach.

course(course_id, title, dept_name, credits)

teaches(ID, course_id, sec_id,semester, year)

instructor(ID,name, dept_name, salary)

Incorrect version

select name, title
from instructor natural join teaches natural join course;

Correct version

select name, title
from instructor natural join teaches, course
where teaches.course_id = course.course_id;

select name, title
from (instructor natural join teacher) join course using(course_id);

select name, title
from instructor，teaches, course
where instructor.ID = teaches .ID and teaches.course_id  = course.course_id;

Example: Find students who takes courses across his/her department.

student( ID, name, dept_name, tot_cred)

takes ( ID, course_id, sec_id, semester, year.grade)

course( course_id, title, dept_name, credits)

answer

select distinct student.id
from (student natural join takes) join course using (course_id)
where student.dept_name <> course.dept_name

The `Rename` Operation¶

The SQL allows renaming relations and attributes using the as clause:

old-name as new-name

select ID, name, salary/12 as monthly_salary
from instructor

Find the names of all instructors who have a higher salary than some instructor in 'Comp. Sci'.

answer

select distinct T. name
from instructor as T, instructor as S
where T.salary > S.salary and S.dept_name = 'Comp. Sci.'

在上述查询中，T 和 S 可以被认为是 instructor 关系的两份拷贝，但更准确地说，它们被声明为 instructor 关系的别名，也就是另外的名称。像T和S那样被用来重命名关系的标识在 SQL标准中被称作相关名称 (correlation name) ，但通常也被称作表别名 (table alias)。或相关变量 (correlation variable) ，或元组变量(tuple variable)。

`String` Operations¶

SQL includes a string-matching operator for comparisons on character strings. The operator like uses patterns that are described using two special characters:

percent (%). The % character matches any substring.
underscore (_). The _ character matches any character.

Example: Find the names of all instructors whose name includes the substring "dar".

answer

select name
from instructor
where name like '%dar%'

Example: Match the string "100 %"

answer

like '100 \%' escape '\' 
like '100 \%' 
like '100 #%' escape '#'

我们在 LIKE 比较运算中使用 ESCAPE 关键字来定义转义字符。

Patters are case sensitive.

Pattern matching examples:

'Intro%' matches any string beginning with "Intro".
'%Comp%' matches any string containing "Comp" as a substring.
'_ _ _ ' matches any string of exactly three characters.
'_ _ _ %' matches any string of at least three characters.

SQL supports a variety of string operations (不同数据库系统所提供的字符串函数集是不同的) such as

concatenation （using ||）用于连接字符串
converting from upper to lower case (and vice versa) 大小写转换(upper(s), lower(s))
去掉字符后面的空格 (trim(s))
finding string length, extracting substrings, etc.

Ordering the Display of Tuples¶

List in alphabetic order the names of all instructors

answer

select distinct name
from instructor
order by name

We may specify desc for descending(降) order or asc for ascending（升） order, for each attribute; ascending order is the default.

Example: order by name desc

Can sort on multiple attributes

Example: order by dept_name, name

查询结果会按照以下规则排序：首先按 dept_name 排序：结果集会根据 dept_name 列的值进行升序排序（默认是升序，即从 A 到 Z）。如果 dept_name 相同，再按 name 排序：在 dept_name 相同的情况下，结果集会根据 name 列的值进行升序排序。

The limit Clause¶

The limit clause can be used to constrain the number of rows returned by the select statement.

limit clause takes one or two numeric arguments, which must both be nonnegative integer constants:

limit offset, row_count
limit row_count == limit 0, row_count

Example: List names of instructors whose salary is among top 3.

answer

select name
from instructor
order by salary desc
limit 3； // limit 0,3

row_count : 限制查询结果返回的行数。
offset : 指定从查询结果的第几行开始返回数据。

Duplicates¶

In relations with duplicates, SQL can define how many copies of tuples appear in the result.

Multiset (多重集) versions of some of the relational algebra operators – given multiset relations $r_1$ and $r_2$ .

Set Operations¶

这些是真正的集合操作，是需要去重的。

(select course_id from section where sem = 'Fall' and year = 2009)
union
(select course_id from section where sem = 'Spring' and year = 2010)

(select course_id from section where sem = 'Fall' and year = 2009)
intersect
(select course_id from section where sem = 'Spring' and year = 2010)

(select course_id from section where sem = 'Fall' and year = 2009)
except
(select course_id from section where sem = 'Spring' and year = 2010)

如果想要不去重，则需要使用 all 。

Suppose a tuple occurs m times in r and n times in s, then, it occurs:

m + n times in r union all s
min(m,n) times in r intersect all s
max(0, m – n) times in r except all s

Null Values¶

null signifies an unknown value or that a value does not exist.

The result of any arithmetic expression involving null is null

Example: 5 + null returns null

The predicate is null can be used to check for null values.

Example: Find all instructors whose salary is null.

answer

select name
from instructor
where salary is null

select sum (salary)
from instructor

Above statement ignores null amounts
Result is null if there is no non-null amount
All aggregate operations except count(*) ignore tuples with null values on the aggregated attributes
Comparisons with null values return the special truth value: unknown.

Three-valued logic using the truth value unknown:

OR

(unknown or true) = true
(unknown or false) = unknown
(unknown or unknown) = unknown

AND

(true and unknown) = unknown
(false and unknown) = false
(unknown and unknown) = unknown

NOT

(not unknown) = unknown

Aggregate Functions¶

Find the names and average salaries of all departments whose average salary is greater than 42000

answer

select dept_name, avg(salary)
from instructor
group by dept_name
having avg(salary) > 42000

having 也是筛选条件，但是是对组进行筛选

about distinct

SQL 不允许在用 count(*) 时使用 distinct 。在用 max 和 min 时使用 distinct 是合法的,尽管结果并无差别。我们可以使用关键字 all 替代 distinct 来表明要保留重复项，但既然 all 是缺省的，就没必要这么做了。

综合版本:

select dept_name, count (*) as cnt
from instructor
where salary >=100000
group by dept_name
having count (*) > 10
order by cnt;

这条 SQL 查询语句的作用是从 instructor 表中筛选出工资 (salary) 大于等于 100,000 的教师，按部门 (dept_name) 分组，统计每个部门的教师人数，并筛选出教师人数大于 10 的部门，最后按教师人数升序排序。

与 select 子句的情况类似，任何出现在 having 子句中，但没有被聚集的属性必须出现在 groupby 子句中，否则查询就是错误的。

包含聚集、group by 或 having 子句的查询的含义可通过下述运算序列来定义:

与不带聚集的查询情况类似，首先根据 from 子句来计算出一个关系。
如果出现了 where 子句，where 子句中的谓词将应用到 from 子句的结果关系上
如果出现了 group by 子句，满足 where 谓词的元组通过 group by 子句被放入分组中。如果没有 group by 子句，满足 where 谓词的整个元组集被当成一个分组。
如果出现了 having 子句，它将应用到每个分组上;不满足 having 子句谓词的分组将被去掉。
select 子句利用剩下的分组产生查询结果中的元组，即在每个分组上应用聚集函数来得到单个结果元组。

易错

SELECT dept_name,ID, avg(salary)
FROM instructor
GROUP BY dept_name;

语句在 SELECT 中包含了 ID 列，但 GROUP BY 仅按 dept_name 分组。ID 既未包含在 GROUP BY 子句中，也未使用聚合函数（如 MAX、MIN），导致数据库无法确定每个分组应返回哪个 ID。

修正思路：

移除 ID 列：如果目标只是查询每个部门的平均工资，直接移除 ID 列即可。

SELECT dept_name, AVG(salary)
FROM instructor
GROUP BY dept_name;

保留 ID 的替代方案：若需要显示部门中的某个 ID（如最大或最小的 ID），需使用聚合函数。

SELECT dept_name, MAX(ID) AS sample_id, AVG(salary)
FROM instructor
GROUP BY dept_name;

Nested Subqueries¶

SQL provides a mechanism for the nesting of subqueries.

SQL 提供了一种嵌套子查询的机制。

A subquery is a select-from-where expression that is nested within another query.

A common use of subqueries is to perform tests for :

set membership
set comparisons
set cardinality

Set Membership¶

Find courses offered in Fall 2009 and in Spring 2010

answer

select distinct course_id
from section
where semester = 'Fall' and year= 2009 and 
course_id in (select course_id
            from section
            where semester = 'Spring' and year= 2010);

真正执行的时候可能会被优化掉。

Find the total number of (distinct) students who have taken course sections taught by the instructor with ID 10101

answer

select count (distinct ID)
from takes
where (course_id, sec_id, semester, year) in
                (select course_id, sec_id, semester, year
                from teaches
                where teaches.ID= '10101');

Set Comparison¶

Find names of instructors with salary greater than that of some (at least one) instructor in the Biology department.

answer

select distinct T.name
from instructor as T, instructor as S
where T.salary > S.salary and S.dept_name = 'Biology';

Same query using > some clause

answer

select name
from instructor
where salary > some (select salary
                    from instructor
                    where dept_name = 'Biology');

Find the names of all instructors whose salary is greater than the salary of all instructors in the Biology department.

answer

select name
from instructor
where salary > all (select salary
                    from instructor
                    where dept_name = 'Biology');

"至少必某一个要大"在 SQL 中用 > some 来表示。SQL 也允许使用 < some、<= some、>= some、= some 和 <> some 的比较。

<> some 并不等价于 not in

对 NULL 值的处理不同

NOT IN 遇到 NULL 会失效。如果子查询的结果中包含 NULL，则 NOT IN 的逻辑会直接失效。

例如:

SELECT * FROM table 
WHERE col NOT IN (SELECT nullable_col FROM subquery);

如果子查询的 nullable_col 存在 NULL，则整个表达式等价于：

col != val1 AND col != val2 AND ... AND col != NULL;

由于 col != NULL 的结果是 UNKNOWN（而不是 TRUE/FALSE），最终整个条件会被判定为 UNKNOWN，导致结果为空。

<> SOME 会忽略 NULL

<> SOME 的语义是“至少存在一个值不等于目标值”，而 NULL 的比较（如 col <> NULL）会被视为 UNKNOWN，不会影响其他有效值的判断。

即使子查询包含 NULL，只要存在至少一个非 NULL 值不等于 col，条件就会成立。

对空子查询的处理不同

NOT IN 的空子查询会返回所有行

如果子查询结果为空，NOT IN 的逻辑等价于 col NOT IN (空集合)，此时 NOT IN 的条件恒为 TRUE，因此会返回所有行。

<> SOME 的空子查询会返回空结果

如果子查询为空，<> SOME 等价于“至少存在一个空集合中的值不等于 col”，由于没有值可以比较，条件恒为 FALSE，因此不会返回任何行。

Scalar Subquery¶

Scalar (标量) subquery is one which is used where a single value is expected.

确认返回只有一个值的时候可以用。否则。 Runtime error if subquery returns more than one result tuple.

select name
from instructor
where salary * 10 > 
    (select budget from department 
    where department.dept_name = instructor.dept_name)

Test for Empty Relations¶

The exists construct returns the value true if the argument subquery is nonempty.

exists $r \leftrightarrow r \neq \emptyset$
not exists $r \leftrightarrow r = \emptyset$

Info

我们一般通过使用 not exists 结构来测试子查询结果集中是否不存在元组。

我们可以使用 not exists 结构来模拟集合包含(即超集)运算:可将"关系A包含关系B"写成not exists(B except A)。

Correlation Variables¶

Yet another way of specifying the query "Find all courses taught in both the Fall 2009 semester and in the Spring 2010 semester"

answer

select course_id
from section as S
where semester = 'Fall' and year = 2009 and 
    exists (select *
        from section as T
        where semester = 'Spring' and year= 2010 
                and S.course_id= T.course_id);

Find all students who have taken all courses offered in the Biology department.

answer

select distinct S.ID, S.name
from student as S
where not exists ((select course_id
                from course
                where dept_name = 'Biology')
                except
                (select T.course_id
                from takes as T
                where S.ID = T.ID));

Note that $X - Y = \emptyset \leftrightarrow X \subseteq Y$

我们可以通过使用 not exists 结构来测试子查询结果集中是否不存在元组。我们可以使用 not exists 结构来模拟集合包含(即超集)运算:可将"关系A包含关系B"写成 not exists(B except A) 。

Note : Cannot write this query using = all and its variants.

Test for Absence of Duplicate Tuples¶

The unique construct tests whether a subquery has any duplicate tuples in its result.

Evaluates to “true” on an empty set

Example: Find all courses that were offered at most once in 2009.

answer

select T.course_id
from course as T
where unique (select R.course_id
            from section as R
            where T.course_id= R.course_id 
                and R.year = 2009) ;

Info

子查询里面不能加 distinct ，只能加在最外面。

Example: Find all courses that were offered once in 2009.

answer

select T.course_id
from course as T
where unique (select R.course_id
            from section as R
            where T.course_id= R.course_id 
                and R.year = 2009) 
        and exists (select R.course_id
            from section as R
            where T.course_id= R.course_id 
                and R.year = 2009) ;

another version

       and course_id in (select course_id
            from section
            where year = 2009) ;

Example: 我们可以使用 not unique 结构测试一个子查询结果中是否存在重复元组。

answer

select T.course_id
from course as T
where not unique (select R.course_id
            from section as R
            where T.course_id= R.course_id 
                and R.year = 2007) ;

With Clause¶

The with clause provides a way of defining a temporary view whose definition is available only to the query in which the with clause occurs.

Example: Find all departments with the maximum budget

answer

with max_budget (value) as 
    (select max(budget)
    from department)
select dept_name
from department, max_budget
where department.budget = max_budget.value;

select dept_name
from department
where budget = (select (max(budget) from department));

with 语句打头先生成一个临时表。

With clause is very useful for writing complex queries.

Example: Find all departments where the total salary is greater than the average of the total salary at all departments.

answer

with dept _total (dept_name, value) as
    (select dept_name, sum(salary)
    from instructor
    group by dept_name),
    dept_total_avg(value) as
    (select avg(value)
    from dept_total)
select dept_name
from dept_total, dept_total_avg
where dept_total.value >= dept_total_avg.value;

Modification of the Database¶

Deletion of tuples from a given relation
Insertion of new tuples into a given relation
Updating values in some tuples in a given relation

Deletion¶

Delete all instructors.

delete from instructor

将删除 instructor 关系中的所有元组。instructor 关系本身仍然存在，但它变成空的了。

Delete all tuples in the instructor relation for those instructors associated with a department located in the Watson building.

delete from instructor
where dept_name in (select dept_name
                    from department
                    where building = 'Watson');

Example: Delete all instructors whose salary is less than the average salary of instructors.

answer

delete from instructor
where salary< (select avg (salary) from instructor);

先算 avg 再删除，否则删除后 avg 就变了。

Insertion¶

Add a new tuple to course.

insert into course
    values ('CS-437', 'Database Systems', 'Comp. Sci.', 4);

or equivalently. 这条可以用于忘记了列的顺序的时候，只要列名和值对应即可。

insert into course (course_id, title, dept_name, credits)
    values ('CS-437', 'Database Systems', 'Comp. Sci.', 4);

更常见的情况是，我们可能想在查询结果的基础上插入元组。

假设我们想让 Music 系每个修满 144 学时的学生成为 Music 系的教师，其工资为 18000 美元。我们可写为:

insert into instructor
select ID, name,dept_name,18000
from student
where dept_name ='Music' and tot_cred > 144;

和本小节前面的示例不同的是，我们用 select 指定了一个元组的集合，而不是指定一个元组。SQL首先执行这条 select 语句，求出将要插入 instructor 关系中的元组集合。每个元组都有 ID、name、dept_name(Music) 和 18000 美元的工资。

系统在执行任何插入之前先执行完 select 语句是非常重要的。如果在执行 select 语句的同时执行某些插入动作，像

insert into student
select *
from student;

只要在 student 上没有主码约束，这样的请求就可能会插入无数元组。如果没有主码约束上述请求会重新插入 student 中的第一个元组，产生该元组的第二份拷贝。由于这第二份拷贝现在成为 student 的一部分，select 语句可能找到它，于是第三份拷贝被插入 student 中。这第三份拷贝又可能被 select 语句发现，于是又插入第四份拷贝，如此等等，无限循环。在执行插入之前先完成 select 语句的执行可以避免这样的问题。这样，如果在 student 关系上没有主码约束，那么上述 insert 语句就只是把 student 关系中的每个元组都复制一遍。

Update¶

Increase salaries of instructors whose salary is over $100,000 by 3%, and all others receive a 5% raise

update instructor
    set salary = salary * 1.03
    where salary > 100000;
update instructor
    set salary = salary * 1.05
    where salary <= 100000;

Same query as before but with case statement

update instructor
    set salary = case
                when salary <= 100000 then salary * 1.05
                else salary * 1.03
                end

标量子查询在 SQL 更新语句中非常有用，它们可以用在 set 子句中。考虑这样一种更新:我们把每个student 元组的 totcred 属性值设为该生成功学完的课程学分的总和。我们假设如果一名学生在某门课程上的成绩既不是"F'也不是空，那么他就成功学完了这门课程。我们需要使用 set 子句中的子查询来写出这种更新，如下所示:

UPDATE student
SET tot_cred =(
    SELECT sum(credits)
    FROM takes, course
    WHERE student.ID = takes.ID
        AND takes.course_id= course.course_id
        AND takes.grade <> 'F'
        AND takes.grade is not null);

如果一名学生没有成功学完任何课程，上述语句将把其 tot_cred 属性值置为空。如果想把这样的属性值置为0，我们可以使用另一条 update 语句来把空值替换为0。

更好的替代方案是把上述子查询中的 select sum(credits) 子句替换为如下使用 case 表达式的 select 子句:

SELECT case
    WHEN sum(credits) is not null then sum(credits)
    else 0
    end

Chapter 3 | Introduction to SQL¶

Data Definition Language¶

Domain Types in SQL¶

Type Conversion and Formatting Functions¶

Default Values¶

Create Table Construct¶

Integrity Constraints in Create Table¶

And a Few More Relation Definitions¶

And more still¶

Drop and Alter Table Constructs¶

SQL and Relational Algebra¶

Basic Query Structure¶

The select Clause¶

The where Clause¶

The from Clause¶

Joins¶

Natural Join¶

The Rename Operation¶

String Operations¶

Ordering the Display of Tuples¶

The limit Clause¶

Duplicates¶

Set Operations¶

Null Values¶

Aggregate Functions¶

Nested Subqueries¶

Set Membership¶

Set Comparison¶

Scalar Subquery¶

Test for Empty Relations¶

Correlation Variables¶

Test for Absence of Duplicate Tuples¶

With Clause¶

Modification of the Database¶

Deletion¶

Insertion¶

Update¶

评论

The `select` Clause¶

The `where` Clause¶

The `from` Clause¶

`Joins`¶

`Natural Join`¶

The `Rename` Operation¶

`String` Operations¶