Introduction to structured query language (sql)

Introduction to Structured Query Language (SQL) CHAPTER 4

Introduction to SQL SQL functions fit into two broad categories: ◦ Data definition language ◦ SQL includes commands to: ◦ Create database objects, such as tables, indexes, and views ◦ Define access rights to those database objects ◦ Data manipulation language ◦ Includes commands to insert, update, delete, and retrieve data within database tables 2

SQL is relatively easy to learn Basic command set has vocabulary of less than 100 words Nonprocedural language American National Standards Institute (ANSI) prescribes a standard SQL Several SQL dialects exist 3

Data Definition Commands Examine simple database model and database tables that will form basis for many SQL examples Understand data environment 7

Creating the Database Following two tasks must be completed: ◦ Create database structure ◦ Create tables that will hold end-user data First task: ◦ RDBMS creates physical files that will hold database ◦ Tends to differ substantially from one RDBMS to another 9

The Database Schema Authentication ◦ Process through which DBMS verifies that only registered users are able to access database ◦ Log on to RDBMS using user ID and password created by database administrator Schema ◦ Group of database objects—such as tables and indexes—that are related to each other 10

Data Types Data type selection is usually dictated by nature of data and by intended use Pay close attention to expected use of attributes for sorting and data retrieval purposes 11

Creating Table Structures Use one line per column (attribute) definition Use spaces to line up attribute characteristics and constraints Table and attribute names are capitalized NOT NULL specification UNIQUE specification 13

Primary key attributes contain both a NOT NULL and a UNIQUE specification RDBMS will automatically enforce referential integrity for foreign keys Command sequence ends with semicolon 14

SQL Constraints NOT NULL constraint ◦Ensures that column does not accept nulls UNIQUE constraint ◦Ensures that all values in column are unique DEFAULT constraint ◦Assigns value to attribute when a new row is added to table CHECK constraint ◦Validates data when attribute value is entered 15

SQL Indexes When primary key is declared, DBMS automatically creates unique index Often need additional indexes Using CREATE INDEX command, SQL indexes can be created on basis of any selected attribute Composite index ◦Index based on two or more attributes ◦Often used to prevent data duplication 16

Data Manipulation Commands Adding table rows Saving table changes Listing table rows Updating table rows Restoring table contents Deleting table rows Inserting table rows with a select subquery 17

Adding Table Rows INSERT ◦ Used to enter data into table ◦ Syntax: ◦ INSERT INTO columnname VALUES (value1, value2, … , valuen); 18

When entering values, notice that: ◦Row contents are entered between parentheses ◦Character and date values are entered between apostrophes ◦Numerical entries are not enclosed in apostrophes ◦Attribute entries are separated by commas ◦A value is required for each column Use NULL for unknown values 19

Saving Table Changes Changes made to table contents are not physically saved on disk until, one of the following occurs: ◦Database is closed ◦Program is closed ◦COMMIT command is used Syntax: ◦COMMIT [WORK]; Will permanently save any changes made to any table in the database 20

Listing Table Rows SELECT ◦ Used to list contents of table ◦ Syntax: ◦ SELECT columnlist FROM tablename; Columnlist represents one or more attributes, separated by commas Asterisk can be used as wildcard character to list all attributes 21

Updating Table Rows UPDATE ◦ Modify data in a table ◦ Syntax: ◦ UPDATE tablename SET columnname = expression [, columname = expression] [WHERE conditionlist]; If more than one attribute is to be updated in row, separate corrections with commas 22

Restoring Table Contents ROLLBACK ◦Used to restore database to its previous condition ◦Only applicable if COMMIT command has not been used to permanently store changes in database Syntax: ◦ROLLBACK; COMMIT and ROLLBACK only work with data manipulation commands that are used to add, modify, or delete table rows 23

Deleting Table Rows DELETE ◦ Deletes a table row ◦ Syntax: ◦ DELETE FROM tablename [WHERE conditionlist ]; WHERE condition is optional If WHERE condition is not specified, all rows from specified table will be deleted 24

Inserting Table Rows with a Select Subquery INSERT ◦ Inserts multiple rows from another table (source) ◦ Uses SELECT subquery ◦ Query that is embedded (or nested) inside another query ◦ Executed first ◦ Syntax: ◦ INSERT INTO tablename SELECT columnlist FROM tablename; 25

Selecting Rows with Conditional Restrictions Select partial table contents by placing restrictions on rows to be included in output ◦ Add conditional restrictions to SELECT statement, using WHERE clause Syntax: ◦ SELECT columnlist FROM tablelist [ WHERE conditionlist ] ; 26

Selecting Rows with Conditional Restrictions (continued) 28

Arithmetic Operators: The Rule of Precedence Perform operations within parentheses Perform power operations Perform multiplications and divisions Perform additions and subtractions 29

Special Operators BETWEEN ◦ Used to check whether attribute value is within a range IS NULL ◦ Used to check whether attribute value is null LIKE ◦ Used to check whether attribute value matches given string pattern 31

IN ◦ Used to check whether attribute value matches any value within a value list EXISTS ◦ Used to check if subquery returns any rows 32

Advanced Data Definition Commands All changes in table structure are made by using ALTER command ◦ Followed by keyword that produces specific change ◦ Following three options are available: ◦ ADD ◦ MODIFY ◦ DROP 33

Changing a Column’s Data Type ALTER can be used to change data type Some RDBMSs (such as Oracle) do not permit changes to data types unless column to be changed is empty 34

Changing a Column’s Data Characteristics Use ALTER to change data characteristics If column to be changed already contains data, changes in column’s characteristics are permitted if those changes do not alter the data type 35

Adding a Column Use ALTER to add column ◦ Do not include the NOT NULL clause for new column 36

Dropping a Column Use ALTER to drop column ◦ Some RDBMSs impose restrictions on the deletion of an attribute 37

Copying Parts of Tables SQL permits copying contents of selected table columns so that the data need not be reentered manually into newly created table(s) First create the PART table structure Next add rows to new PART table using PRODUCT table rows 39

Adding Primary and Foreign Key Designations When table is copied, integrity rules do not copy, so primary and foreign keys need to be manually defined on new table User ALTER TABLE command ◦ Syntax: ◦ ALTER TABLE tablename ADD PRIMARY KEY(fieldname); ◦ For foreign key, use FOREIGN KEY in place of PRIMARY KEY 40

Deleting a Table from the Database DROP ◦ Deletes table from database ◦ Syntax: ◦ DROP TABLE tablename; 41

Advanced Select Queries SQL provides useful functions that can: ◦ Count ◦ Find minimum and maximum values ◦ Calculate averages SQL allows user to limit queries to only those entries having no duplicates or entries whose duplicates may be grouped 42

Virtual Tables: Creating a View View is virtual table based on SELECT query ◦ Can contain columns, computed columns, aliases, and aggregate functions from one or more tables Base tables are tables on which view is based Create view by using CREATE VIEW command 51

Joining Database Tables Ability to combine (join) tables on common attributes is most important distinction between relational database and other databases Join is performed when data are retrieved from more than one table at a time Join is generally composed of an equality comparison between foreign key and primary key of related tables 53

Joining Tables with an Alias Alias can be used to identify source table Any legal table name can be used as alias Add alias after table name in FROM clause ◦ FROM tablename alias 54

Summary SQL commands can be divided into two overall categories: ◦ Data definition language commands ◦ Data manipulation language commands The ANSI standard data types are supported by all RDBMS vendors in different ways Basic data definition commands allow you to create tables, indexes, and views 55

DML commands allow you to add, modify, and delete rows from tables The basic DML commands are SELECT, INSERT, UPDATE, DELETE, COMMIT, and ROLLBACK INSERT command is used to add new rows to tables SELECT statement is main data retrieval command in SQL 56

Many SQL constraints can be used with columns The column list represents one or more column names separated by commas WHERE clause can be used with SELECT, UPDATE, and DELETE statements to restrict rows affected by the DDL command 57

Aggregate functions ◦ Special functions that perform arithmetic computations over a set of rows ORDER BY clause ◦ Used to sort output of SELECT statement ◦ Can sort by one or more columns and use either an ascending or descending order Join output of multiple tables with SELECT statement 58

Natural join uses join condition to match only rows with equal values in specified columns Right outer join and left outer join used to select rows that have no matching values in other related table 59

Introduction to structured query language (sql)

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Introduction to structured query language (sql)