Frequently used SQL DDL statements

A collection of reference examples of SQL DDL (Data Definition Language) statements frequently used in application development to create and alter database tables, synonyms, constraints, indexes etc. 
Note: The statements have been tested on Oracle 11G database; most probably should work with other relational databases. Some of the datatypes used in the samples are Oracle-specific and would require some tweaking.

-- Create a table
CREATE TABLE TX_HISTORY (
  tx_id      NUMBER(10, 0) NOT NULL,
  stock_id   NUMBER(10, 0) NOT NULL,
  tx_date    DATE          NOT NULL,
  currency   VARCHAR2(5)   NOT NULL,
  price      NUMBER(9, 2)  NOT NULL,
  price_2    NUMBER
)
/

-- Create a table with a PK (Primary Key) on a single column
CREATE TABLE STOCK (
  stock_id      NUMBER(10, 0) NOT NULL,
  symbol        VARCHAR2(5)   NOT NULL,
  stk_exchange  VARCHAR2(5)   NOT NULL,
  currency      VARCHAR2(5)   NOT NULL,
--  
  CONSTRAINT stock_id_pk PRIMARY KEY (stock_id)
)
/

-- Add a PK (Primary Key) to already existing table
ALTER TABLE tx_history ADD CONSTRAINT tx_history_pk PRIMARY KEY (tx_id)
/

-- Create a table with a FK (Foreign Key) to another table
CREATE TABLE STOCK_HISTORY (
  rec_id        NUMBER(10, 0) NOT NULL,
  stock_id      NUMBER(10, 0) NOT NULL, -- FK to STOCK table
  trade_date    DATE          NOT NULL,
  open_price    NUMBER(9, 2)  NOT NULL,
  close_price   NUMBER(9, 2)  NOT NULL,
--  
  CONSTRAINT stock_history_stock_fk FOREIGN KEY (stock_id) REFERENCES stock (stock_id) ENABLE VALIDATE
)
/

-- Add a FK (Foreign Key) to already existing table to reference another table
ALTER TABLE tx_history ADD CONSTRAINT tx_history_stock_fk FOREIGN KEY (stock_id) REFERENCES stock (stock_id) ENABLE VALIDATE
/
-- Drop (delete) a table
DROP TABLE TX_HISTORY
/

-- Add a new column to an existing table
ALTER TABLE TX_HISTORY ADD QTY NUMBER
/

-- Drop a column
ALTER TABLE TX_HISTORY DROP COLUMN PRICE_2
/