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More SQL

Database ModificationDefining a Database SchemaViews

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Database Modifications

A modification command does not return a result (as a query does), but changes the database in some way.

Three kinds of modifications:1. Insert a tuple or tuples.

2. Delete a tuple or tuples.

3. Update the value(s) of an existing tuple or tuples.

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Insertion

To insert a single tuple:

INSERT INTO <relation>

VALUES ( <list of values> ); Example: add Sydney Greenstreet to the list of

stars of The Maltese Falcon.

INSERT INTO StarsInVALUES(‘The Maltese Falcon’, 1942, ’Sydney GreenStreet’);

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Specifying Attributes in INSERT

We may add to the relation name a list of attributes.

Two reasons to do so:1. We forget the standard order of attributes for the

relation.

2. We don’t have values for all attributes, and we want the system to fill in missing components with NULL or a default value.

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Example: Specifying Attributes

Another way to add Sydney Greenstreet to the list of stars of The Maltese Falcon.

INSERT INTO StarsIn(movieTitle, movieYear, starName)VALUES(’The Maltese Falcon’, 1942, ’Sydney GreenStreet’);

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Inserting Many Tuples

We may insert the entire result of a query into a relation, using the form:

INSERT INTO <relation>

( <subquery> );

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Example: Insert a Subquery

Using Studio and Movie, add to the relation Studio all movie studios that are mentioned in the relation Movie, but don’t appear in Studio.

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Solution

INSERT INTO Studio(name)

(SELECT DISTINCT studioName

FROM Movie

WHERE studioName NOT IN

(SELECT name

FROM Studio));

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Deletion

To delete tuples satisfying a condition from some relation:

DELETE FROM <relation>

WHERE <condition>;

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Example: Deletion

Delete from relation StarsIn the fact that Sydney GreenStreet was a star in The Maltese Falcon:

DELETE FROM StarsInWHERE movieTitle = ‘The Maltese Falcon’

AND movieYear = 1942 AND starName = ‘Sydney Greenstreet’;

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Example: Delete all Tuples

Make the relation Likes empty:

DELETE FROM Likes;

Note no WHERE clause needed.

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Example: Delete Many Tuples

Delete from MovieExec all movie executives whose net worth is low-less than ten million dollars.

DELETE FROM MovieExec

WHERE netWorth < 10000000;

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Updates

To change certain attributes in certain tuples of a relation:

UPDATE <relation>

SET <list of attribute assignments>

WHERE <condition on tuples>;

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Example: Update

Modify the relation MovieExec by prepending the title Pres. In front of every movie executives who is the president of a studio:

UPDATE MovieExec

SET name = ‘Pres. ’ || name

WHERE cert# IN

(SELECT presC# FROM Studio);

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Defining a Database Schema

A database schema comprises declarations for the relations (“tables”) of the database.

Several other kinds of elements also may appear in the database schema, including views, indexes, and triggers, which we’ll introduce later.

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Creating (Declaring) a Relation

Simplest form is:

CREATE TABLE <name> (

<list of elements>

); To delete a relation:

DROP TABLE <name>;

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Elements of Table Declarations

Most basic element: an attribute and its type. The most common types are:

INT or INTEGER REAL or FLOAT CHAR(n )

• fixed-length string of n characters.

VARCHAR(n )• variable-length string of up to n characters.

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Example: Create Table

CREATE TABLE MovieStar (

name CHAR(30),

address VARCHAR(255),

gender CHAR(1),

birthdate DATE

);

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Dates and Times

DATE and TIME are types in SQL. The form of a date value is:

DATE ’yyyy-mm-dd’ Example: DATE ’2004-09-30’

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Times as Values

The form of a time value is:

TIME ‘hh:mm:ss’

with an optional decimal point and fractions of a second following. Example: TIME ‘15:30:02.5’

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Modifying relation schemas

We can use ALTER to modify a relation schema. We have several options, the most important of which are: ADD followed by a column name and its data

type; DROP followed by a column name;

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Adding Attributes

We may add a new attribute (“column”) to a relation schema by:

ALTER TABLE <name> ADD

<attribute declaration>; Example:

ALTER TABLE MovieStar ADD phone CHAR(16);

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Deleting Attributes

Remove an attribute from a relation schema by:

ALTER TABLE <name>

DROP <attribute>; Example: we don’t really need the license

attribute for bars:

ALTER TABLE MovieStar DROP birthdate;

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Default values

When we create or modify tuples, we sometimes don’t have values for all components.

To address this problem, SQL provides the NULL value.

However, there are times when we would prefer to use default value, the value that is placed in a component if no other value is known.

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Example

We might wish to use the character ‘?’ as the default for an unknown gender, and we might also wish to use the earliest possible date, DATE ‘0000-00-00’ for an unknown birthdate. CREATE TABLE MovieStar (

name CHAR(30),address VARCHAR(255),gender CHAR(1) DEFAULT ‘?’,birthdate DATE DEFAULT DATE ‘0000-00-00’);

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Indexes

An index on an attribute A of a relation is a data structure that makes it efficient to find those tuples that have a fixed value for attribute A.

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To create a index

Create an index on attribute year for the relation MovieCREATE INDEX YearIndex ON Movie(year);

From Movie, create an index on title and yearCREATE INDEX KeyIndex ON Movie(title,

year);

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To delete a index

If we wish to delete the index, we simply use its name in a statement like:DROP INDEX YearIndex;

Selection of indexes requires a trade-off by the database designer The existence of an index on an attribute greatly

speeds up queries in which a value for that attribute is specified.

On the other hand, ervery index built for an attribute of some relation makes insertions, deletion, and updates to that relation more complex and time-consuming.

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Views

A view is a “virtual table” = a relation defined in terms of the contents of other tables and views.

Declare by:

CREATE VIEW <name> AS <query>; Antonym: a relation whose value is really

stored in the database is called a base table.

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Example: View Definition

To define a view that is a part of the Movie relation, specifically, the titles and years of the movies made by Paramount Studio:

CREATE VIEW ParamountMovie AS

SELECT title, year

FROM Movie

WHERE studioName = ‘Paramount’;

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Example: Accessing a View

Query a view as if it were a base table. Also: a limited ability to modify views if it

makes sense as a modification of one underlying base table.

Example query:

SELECT title

FROM ParamountMovie

WHERE year = 1979;

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What Happens When a View Is Used?

The SQL system will translate the query on the view ParamountMovie into a query about the base table Movie that has the same effect as our original query.SELECT titleFROM MovieWHERE studioName = ‘Paramount’ AND year = 1979;

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Define a query based on views and base tables

Example:SELECT DISTINCT starName

FROM ParamountMovie, StarsIn

WHERE title = movieTitle AND year = movieYear;

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Renaming attributes

We can give a view’s attributes names of our own choosing. For example:CREATE VIEW MovieProd(movieTitle, prodName)

AS

SELECT title, name

FROM Movie, MovieExec

WHERE producerC# = cert#;

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Delete a view

If a view becomes unuseful, we can delete it. For instance:DROP VIEW ParamountMovie;

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视图的作用 简化用户操作

数据看起来简单、清晰。 用户可以从多角度看待同一数据

灵活共享数据库。 对数据库重构提供了一定程度的逻辑独立性

当数据库重构造 ( 如增加表，或增加字段 ) 时，用户和用户程序不会受影响。只是由于视图的更新是有条件的，因此视图只能在一定程度上提供数据的逻辑独立性。

能够对机密数据提供安全保护 对不同的用户定义不同的视图，实现重要数据的隐藏。

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练习职工 - 工资数据库职工（工号，姓名，职务，部门）工资表（工号，月份，基本工资，岗位津贴，交通补贴）

建立职工工号和每月基本工资的视图 建立每个职工的姓名和他当月总收入的视图 建立本月总收入低于 1000元的职工的视图

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NULL Values

Tuples in SQL relations can have NULL as a value for one or more components.

Meaning depends on context. Two common cases: Missing value : e.g., we know Joe’s Bar has

some address, but we don’t know what it is. Inapplicable : e.g., the value of attribute spouse

for an unmarried person.

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Two important rules

When we operate on a NULL and any other value, including another NULL, using an arithmetic operator like × or +, the result is NULL.

When we compare a NULL value and any value, including another NULL, using a comparison operator like = or >, the result is UNKNOWN. The value UNKNOWN is another truth-value, like TRUE and FALSE.

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To ask if x has the value NULL

x IS NULL, this expression have the value TRUE if x has the value NULL and it has FALSE otherwise.

x IS NOT NULL, this expression have the value FALSE if x has the value NULL and it has TRUE otherwise

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Comparing NULL’s to Values

The logic of conditions in SQL is really 3-valued logic: TRUE, FALSE, UNKNOWN.

But a query only produces a tuple in the answer if its truth value for the WHERE clause is TRUE (not FALSE or UNKNOWN).

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Join Expressions

SQL provides several versions of joins. These expressions can be stand-alone

queries or used in place of relations in a FROM clause.

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Products and Natural Joins

Natural join:

R NATURAL JOIN S; Product:

R CROSS JOIN S; Example:

Likes NATURAL JOIN Serves; Relations can be parenthesized subqueries, as well.

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Theta Join

R JOIN S ON <condition> Example: using Movie and StarIn:

Movie JOIN StarIn ON

title = movieTitle AND year = movieyear;

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Outerjoins

R OUTER JOIN S is the core of an outerjoin expression. It is modified by:

1. Optional NATURAL in front of OUTER.

2. Optional ON <condition> after JOIN.

3. Optional LEFT, RIGHT, or FULL before OUTER. LEFT = pad dangling tuples of R only. RIGHT = pad dangling tuples of S only. FULL = pad both; this choice is the default.

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Example

MovieStar NATURAL FULL OUTER JOIN MovieExec; Where FULL can be replaced by LEFT or

RIGHT

Movie FULL OUTER JOIN StarIn ON title = movieTitle AND year = movieYear; Where FULL can be replaced by LEFT or

RIGHT