Data Definition

Data-Definition

  • Data definition language defines
  • Databases
  • Tables
  • Attributes and their types
  • Constraints
CREATE TABLE employees (
    employee_id INT PRIMARY KEY,
    first_name VARCHAR(50) NOT NULL,
    last_name VARCHAR(50) NOT NULL,
    department VARCHAR(100),
    salary DECIMAL(10, 2) NOT NULL
);
CREATE TABLE table_name (
    column1 datatype constraints,
    column2 datatype constraints,
    ...
    PRIMARY KEY (column_name(s)),
    FOREIGN KEY (column_name) REFERENCES other_table_name(column_name)
);
ALTER TABLE borrow ADD FOREIGN KEY (ISBN) REFERENCES books(ISBN)

CREATE

  • create database
CREATE DATABASE uic_example
  • create table
CREATE TABLE table_name (
  attribute1 type1,
  attribute2 type2,
  
  constraint1, 
  constraint2,
  
)

“table_name” is the name of the table. “attribute” is the name of the attribute. “type” is the data type for the attribute. “constraint” is a condition on the table. If users try to insert some data violating the condition, the system will give a warning.

Data Type

Data Type Description
char(n) fixed length character string of at most length n.
varchar(n) variable length character string of at most length n.
int integer (a finite subset of the integers that is - machine-dependent).
smallint small integer (a machine-dependent subset of the integer - domain type).
numeric(p,n) fixed point number, with user-specified precision of p digits, and with n digits to the right of decimal point.
real floating point, with machine-dependent precision.
float(n) floating point number, with user-specified precision of at least n digits.
year 4-digit string or 4-digit number from 1901 to 2155.
date in ‘YYYY-MM-DD’ format from 1000-01-01 to 9999-12-31.
time in ‘HH:MM:SS’ format.
blob binary large object, usually for images.
clob character large object, usually for long text.

VARCHAR 不定长 CHAR 定长

Constraints

  • PRIMARY KEY (attribute) One or multiple attribute(s) Uniquely identify the tuples Unique for each table Cannot be a NULL value by default
  • NOT NULL The value of the attribute cannot be unknown. The condition is checked when rows are inserted into the table.

Foreign key

FOREIGN KEY (attribute(s)1) REFERENCES table2(attribute(s)2)
  • “attribute(s)1” in the current table and “attribute(s)2” in “table2” represent the same piece of information.
  • “attribute(s)2” is the primary key for “table2”.
  • A value of “attribute(s)1” in the current table cannot exist alone.

  • To add foreign keys to an existing table, we need to change the schema.
ALTER TABLE borrow ADD FOREIGN KEY (ISBN) REFERENCES books(ISBN)

ALTER TABLE: the keyword to change the schema of a table. borrow: the table name. ADD: to add new attributes or constraints (“DROP” to remove attributes or constraints). FOREIGN KEY: the constraint. (ISBN): the foreign key. REFERENCES: the keyword to indicate which attribute is the target. books(ISBN): the attribute “ISBN” in the table “books” is referenced.

BackLink