Database Queries

Select Commands

SELECT * from tableName
 - SELECT all columns FROM the table

SELECT column1, column2 FROM tableName

SELECT 2 columns FROM the tableName table

SELECT DISTINCT column FROM tableName

will display only unique results (no duplicates)

SELECT firstname || ' ' || lastname FROM tablename

will concatenate first and last name into one field (the ' ' between the || inserts a space)

SELECT fristname || ' ' || lastname "Customer Name" FROM table name

this will add a alias for the column heading called Customer Name

SELECT title AS "Title of Book", category FROM books;

This adds the column alias "Title of Book" to the results instead of using just the title as the column heading

select table_name FROM user_tables

that exact statement will list all tables

selecting column names, data type, and default values

select column_name, data_type, data_default from user_tab_columns where table_name = 'ACCTMANAGER'

Creating Calculated Fields in a Query

SELECT title, retail*cost as profit FROM books;
retail column is multiplied by cost and the result is displayed in the alias field profit

Creating a table based on another table (subquery)

CREATE TABLE cust_mkt AS (SELECT customers, city, state, zip FROM customers);

SELECT firstname || lastname FROM customers;
this will display the info but last and first names will be together eg. firstlast
to add a space between the names :
SELECT firstname || ' , ' | || lastname as "Customer Name" FROM customers;
this will add the , between the names and give it an alias heading "Customer name"

Adding Primary Key Constraints

This makes customer# the primary key  the constraint name is used so you can easily 
identify an error if you try to enter a duplicate customer
If you don’t assign constraint names, the error message displays the
system-generated constraint name, which isn’t as helpful
ALTER TABLE customers ADD CONSTRAINT customers_customer#_pk PRIMARY KEY(customer#);

Foreign Key Constraint

This command instructs Oracle 11g to add a FOREIGN KEY constraint on the Customer# column of the ORDERS table. The name chosen for the constraint is orders_customer#_fk. This constraint makes sure an entry for the Customer# column of the ORDERS table matches a value stored in the Customer# column of the CUSTOMERS table. When the command executes, a message indicates the table was altered successfully

ALTER TABLE orders ADD CONSTRAINT orders_customer#_fk FOREIGN KEY(customer#) REFERENCE customers (customer#)

Unique Constraints

ALTER TABLE tablename ADD CONSTRAINT constraintname UNIQUE(columnname);

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