Database Systems

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Creating and Maintaining

Database Objects

Part 2

Database Systems

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To find a date that is a specific number of days before or after a known date, add or subtract the number from the known date

Example:SELECT order_date + 30 FROM cust_order;

Date Arithmetic

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To find the number of days between two known dates, subtract the later date from the earlier date

Example:SELECT SYSDATE – s_dobFROM my_students;

Date Arithmetic

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ADD_MONTHS returns a date that is a specific

number of months after a given date

Example:SELECT ADD_MONTHS(SYSDATE, 6)

FROM dual;

Date Functions

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LAST_DATE Returns the date that is the last

day of the month specified in the current date

Example:SELECT LAST_DATE(order_date) FROM cust_orderWHERE order_id = 1057;

Date Functions

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MONTHS_BETWEEN Returns the number of months

between two input dates

Example:SELECT MONTHS_BETWEEN(order_date, SYSDATE)

FROM cust_orderWHERE order_id = 1057;

Date Functions

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Used to perform an operation on a field from a group of retrieved records AVG (average of all retrieved values) COUNT (number of records retrieved) MAX (maximum value retrieved) MIN (minimum value retrieved) SUM (sum of all retrieved values)

Group Functions

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SELECT AVG (s_age) FROM my_students;

SELECT MAX (s_age) FROM my_students;

SELECT MIN (s_age) FROM my_students;

SELECT SUM (s_age) FROM my_students;

Group Function Examples

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GROUP BY must be used if some columns in the SELECT clause are used in a group function and some are not

Group all fields that are not included in the group function

Example:SELECT s_class, AVG(s_age)

FROM my_students

GROUP BY s_class;

Using the GROUP BY Clause

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Creating Alternate Column Headings in SQL*Plus

• Syntax:SELECT column1 “heading1”, column2 “heading2”, …

• Example:SELECT (SYSDATE – s_dob) “Student Age”

FROM my_students;

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Creating a Column Alias

• Column alias: alternate column name that can be referenced in the ORDER BY and GROUP BY clauses

• Syntax:SELECT column1 AS alias1 …

Example:SELECT (SYSDATE – s_dob) AS age_alias

ORDER BY age_alias

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Dynamic SQL Queries

• Queries that allow users to specify search conditions at runtime

• Approaches• Substitution Values• Runtime Variables

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Using Substitution Values

• Created when search expression is prefaced with an ampersand (&)

• System then prompts user for value

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Using Runtime Variables

• Runtime variable: variable defined in SQL*Plus environment• Syntax:

DEFINE variable_name = variable_value;

• You can then substitute the variable name for a query search condition value

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Using Runtime Variables

• Example:

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Formatting Data Using theTO_CHAR Function

• Used to display NUMBER and DATE values using a specific format mask

• Syntax:TO_CHAR(fieldname, ‘format_mask’);

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Join Queries

• Retrieve data from multiple tables by joining tables using foreign key references

• Join query types:• Inner (equality)• Outer• Self• Inequality

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Inner Joins

• One record is retrieved for each matching row

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Syntax:SELECT column1, column2, …

FROM table1, table2

WHERE table1.join_column =

table2.join_column

You must include a join condition for every link between 2 tables

Inner Joins

Join condition

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Example:SELECT s_name, f_nameFROM student, facultyWHERE student.f_id = faculty.f_id;

If you have N tables in the FROM clause, you must have (N - 1) join conditions

Inner Joins

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Qualifying Field Names

• If a field in the SELECT clause exists in multiple tables in the FROM clause, you must qualify the field name by prefacing it with either table’s name

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1. Identify all of the tables involved in the query, and label:

Display fields Join fields Search fields

2. Write the query List all display fields in the SELECT clause List all table names in the FROM clause List all join condition links in the WHERE

clause List all search fields in the WHERE clause

Process for DesigningComplex Inner Join Queries

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Outer Joins

• Limitation of inner joins: some records may be omitted if corresponding records don’t exist in one of the tables

• Example: retrieve records for all students, along with their corresponding ENROLLMENT information

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Outer Joins

Student 105 (Michael Connoly) does not have any ENROLLMENT records

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Outer Joins

• No records retrieved for Michael:

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Outer Joins

To include records in first (inner) table, even when they do not have matching records in second (outer) table, place outer join marker (+) beside outer table name in join clause

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Outer JoinsOuter join marker

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Self Joins

• Used to join a table to itself when the table has a hierarchical relationship

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Self Joins

• To create a self-join, you need to create a table alias, which gives an alternate name to the table so you can create a join condition

• Syntax to create table alias in FROM clause:

FROM table1 alias1, table2 alias2

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Self Joins P_ID PROJECT_NAME CLIENT_ID MGR_ID PARENT_P_ID

1 Hardware Support Intranet 2 105

2 Hardware Support Interface 2 103 1

3 Hardware Support Database 2 102 1

4 Teller Support System 4 105

5 Internet Advertising 6 105

6 Network Design 6 104 5

7 Exploration Database 5 102

PARENT_PROJECT

P_ID PROJECT_NAME CLIENT_ID MGR_ID PARENT_P_ID








PROJECT

P_ID PROJECT_NAME CLIENT_ID MGR_ID PARENT_P_ID








SUB_PROJECT

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Self Join Example

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Inequality Joins

• Join created by placing making join condition satisfy an inequality condition

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Inequality Joins

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Nested Queries

• Created when a sub-query is nested within a main query• Main query: first query listed in

SELECT command• Sub-query: retrieves one or more

values that specify the main query’s search condition

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Nested Query WhereSub-query Returns a Single Value

• Syntax:SELECT column1, column2, …

FROM table1, table2, …

WHERE join conditions

AND search_column1 = (SELECT column1


WHERE search and

join conditions)

Sub-querythat returnsone value

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Nested Query WhereSub-query Returns Multiple Values



WHERE join conditions

AND search_column1 IN (SELECT column1


WHERE search and

join conditions)

Sub-querythat returnsmultiple values

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• Performs set operations on outputs of two unrelated queries

• Both queries must have:• same number of display fields• corresponding display fields must have

same data type

Using Set Operators in Queries

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• UNION: combines results, suppresses duplicate rows

• UNION ALL: combines results, displays duplicates

• INTERSECT: finds matching rows• MINUS: returns the difference

between returned record sets

Query Set Operators

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Selecting Records For Update

• In a normal SELECT command, the retrieved records are not locked, and are available for other users to view, updated, and delete

• Sometimes, you need to select records, and then immediately update them based on the retrieved values• Airline seat reservations• Inventory items for sale

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WHERE search and join conditions

FOR UPDATE OF column1, column2, …

NOWAIT;

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• All retrieved records are locked until you issue a COMMIT command• Fields listed in FOR UPDATE clause are

for documentation purposes only

• NOWAIT clause is optional• Makes it so when another user tries to

retrieved locked record, their system doesn’t just “hang”

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• Logical table based on a query• Does not physically exist in the

database• Presents data in a different format

from underlying tables• Uses:

• Security• Simplifying complex queries

Database Views

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• Creating a view:CREATE VIEW view_name AS

SQL_command;

• Views can be queried just like tables:SELECT *

FROM view_name;

Database Views

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Simple Views

• Based on SQL query that retrieves data from only one table

• View can support all table operations:• INSERT• UPDATE• DELETE

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Complex Views

• Based on query that retrieves data from multiple tables

• Can only be used to support SELECT operations• No table operations supported

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Indexes

• Index: Separate table is maintained that shows index keys and physical locations of corresponding records• In Oracle, ROWID is

translated to physical location of row on disk

• Improves response time of searches and joins

SLName ROWID

Brown 13387289

Jones 13879872

Smith 58925789

Helgeson 29875018

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Using Indexes

• Create table index AFTER table is populated with data• Indexes make INSERT, UPDATE, and DELETE

operations slower because index must also be maintained

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Indexing Strategies

• A table can have indexes on multiple fields• Create indexes based on fields used for search or join

operations• Typically, indexes only speed retrievals when <15% of the

table records are involved

• Each additional index adds processing overhead for INSERT, UPDATE, and DELETE operations

• In Oracle, primary keys are automatically indexed

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Creating Indexes

• Syntax:CREATE INDEX index_name

ON tablename(index_field);

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Synonyms

• Alternate name for a table• Allows you to not have to preface

table with owner’s username when you are querying a table that belongs to another user

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Public Synonyms

• Can only be created by a DBA• Syntax:CREATE PUBLIC SYNONYM synonym_name

FOR owner_name.tablename;

• All users with privileges to use table can then use synonym instead of owner_name.tablename

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Private Synonyms

• You can create private synonyms for any tables that you have privileges to use

• Only you can use the synonym• Syntax:CREATE SYNONYM synonym_name

FOR table_name.table_name;

End of Lecture

Database Systems

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