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DBS201: More on SQL

Lecture 3

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Agenda

How to use SQL to update table definitions How to update data in a table How to join tables together

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Updating a Table Definition

ALTER TABLES Used to update a database definition

Syntax ALTER TABLE tablename

Can do any of the following Add a field Alter a field Drop a field Add a constraint Drop a constraint

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Updating a Table Definition

To add a field: ALTER TABLE tablename ADD COLUMN field-

name datatype i.e.

ALTER TABLE MARINA

ADD COLUMN SLIP_DESCRIPTION CHAR (20)

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Updating a Table Definition

To alter a field: ALTER TABLE tablename ALTER COLUMN

fieldname SET DATA TYPE data type i.e.

ALTER TABLE MARINA

ALTER COLUMN SLIP_DESCRIPTION

SET DATA TYPE CHAR(12)

NOT NULL WITH DEFAULT 'abc'

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Updating a Table Definition

To remove a field: ALTER TABLE tablename DROP COLUMN

fieldname i.e.

ALTER TABLE MARINA

DROP COLUMN SLIP_DESCRIPTION

*Note – be careful not to drop a column that you may in fact need as the data will be lost.

Rule of thumb: do not alter a table after it contains data

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Updating a Table Definition

To add a constraint: ALTER TABLE tablename ADD constraint

Constraint is primary key or foreign key i.e.

ALTER TABLE MARINA ADD constraint Slip_pk PRIMARY KEY (SLIP_DESCRIPTION)

ALTER TABLE tablename ADD constraint Constraint is value restricted i.e.

ALTER TABLE MARINA ADD constraint Slip_ck CHECK (SLIP_DESCRIPTION BETWEEN 'AAA' AND 'DDD‘)

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Updating a Table Definition

To drop a constraint ALTER TABLE tablename

DROP CONSTRAINT constraint name i.e.

ALTER TABLE MARINA

DROP CONSTRAINT slip_ck

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Updating Data in a Table

To update data in a table, use the UPDATE statement UPDATE tablename SET fieldname = new value

WHERE condition i.e.

UPDATE MARINA SET SLIP_DESCRIPTION = ‘AAA'

UPDATE MARINA SET SLIP_DESCRIPTION = ‘AAB‘ WHERE MARINA_ID = ‘AZ24’

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How to Join Tables

Can join two or more tables together in a Select statement

Result is output that has associated the correct rows from each table

Result exists only for the duration of the execution of the query

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How to Join Tables

To create a join, specify the tables that you want to be included

For each field you want, specify it’s full name:

table-name.field-name Must use the WHERE clause of the SELECT

statement. The WHERE clause controls what is being joined

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How to Join Tables

Two tables: Marina and Customer Create a report that shows the boat name and

customer name SELECT statement might look like:

SELECT CUSTOMER.FIRST_NAME, CUSTOMER.LAST_NAME, MARINA.BOAT_NAME FROM CUSTOMER, MARINA WHERE CUSTOMER.OWNER_NUM = MARINA.OWNER_NUM)