constraint

Constraints are applied to the rules on the data in the table column, the type data table limits. There is a constraint ensures accuracy and reliability data in the database.
Constraint can be a column or table level, column-level constraints applied to a single column, table-level constraints applied to the entire data table.

SQLite common constraints:

NOT NULL constraints: Make sure that a column can not have NULL values.
DEFAULT constraint: when a column value is not inserted, provide default values for that column.
UNIQUE constraint: to ensure that all values in a column are different.
PRIMARY KEY constraints: data that uniquely identifies a database table.
FOREIGN KEY constraint: a table pointing FOREIGN KEY PRIMARY KEY in another table.
CHECK constraint: to ensure that all values satisfy certain conditions in a column.

NOT NULL constraints

By default, the column can hold NULL values. If you do not want a column has a NULL value, you need to define this constraint on the column designated NULL values are not allowed on this column.

NULL and no data is not the same, it represents the unknown data.

-- 实例
CREATE TABLE link_men (
    ID INT PRIMARY KEY      NOT NULL;
    NAME           TEXT     NOT NULL
)

DEFAULT constraint

DEFAULT constraint in the INSERT INTO statement does not provide a specific value for the column to provide a default value.

-- 实例
CREATE TABLE link_men(
    ID INT PRIMARY KEY      NOT NULL;
    NAME           TEXT     NOT NULL;
    ADDRESS        TEXT     NOT NULL;
    SALARY         REAL     DEFAULT 8000.00
)

UNIQUE constraint

UNIQUE constraint prevents the presence of two records with the same value in a particular column.

-- 实例
CREATE TABLE link_men(
    ID INT PRIMARY KEY      NOT NULL;
    NAME           TEXT     NOT NULL;
    AGE            INT      NOT NULL UNIQUE;
    ADDRESS        TEXT;
    SALARY        REAL DEFAULT 8000.00
)

PRIMAY KEY constraint

PRIMARY KEY constraint uniquely identifies each record in the database table.
Primary keys must contain unique values.
Primary key column can not contain a NULL value.
Each table should have a primary key, and each table can have only one primary key.

-- 实例
CREATE TABLE link_men(
    ID INT PRIMARY KEY      NOT NULL;
    NAME           TEXT     NOT NULL;
    AGE            INT      NOT NULL UNIQUE;
    ADDRESS        TEXT;
    SALARY       , REAL DEFAULT 8000.00
)

FOREIGN KEY constraint

A table pointing FOREIGN KEY PRIMARY KEY in another table.

-- 实例
CREATE TABLE link_men (
    ID INT PRIMARY KEY     NOT NULL;
    NAME           TEXT    NOT NULL;
    AGE            INT     NOT NULL UNIQUE;
    ADDRESS        TEXT;
    SALARY         REAL DEFAULT 8000.00;
    FOREIGN KEY (P_ID) REFERENCES persons(P_ID);
)

CHECK constraints

CHECK constraints enable input a record you want to check the condition values. If the condition is false, the record violates the constraint, and can not be entered into the table.

CREATE TABLE link_men(
   ID INT PRIMARY KEY     NOT NULL,
   NAME           TEXT    NOT NULL,
   AGE            INT     NOT NULL,
   ADDRESS        TEXT,
   SALARY         REAL    CHECK(SALARY > 0)
);

SQLite Advanced -10. Constraints