Data storage--sqlite Database storage 2

I. Constraints on tables

1. Unique constraints

1-field content (Multi-field content merge) Unique, cannot have duplicate rows

2-the same table can have multiple unique constraints

2. Check constraints

1-Ensure that the requirements are met before saving

2-Validation of the range of values for field data

3. Maintaining integrity between tables

3. FOREIGN KEY constraints

1-The primary key is associated to a field in another table, which is the foreign key

2-Requirement: The foreign key is added or modified, the data must exist in the primary key table; When the primary key field is modified or deleted, the data does not exist in the external key table.

Two. Rules for building tables

Conforms to three paradigms:

1. First paradigm: The meaning of field expression is single, non-split

2. Second paradigm: Avoid data redundancy, split redundant data into multiple tables, and correlate between tables

3. Third paradigm: avoiding transmission of dependence

4. Balance the paradigm and redundancy to achieve the best results for increased efficiency

Three. SQL statements

1. Classification

1-Data Definition language DDL

2-Data Control Language DCL

3-Data Management language DML

4-Data Query Language DQL

2. Operators

1-Arithmetic operator: + 、-、 *,/

2-Comparison operators: >,>=,<,<=,=,!=, <>

3-logical operators: and with, or or, not non-

4-string Operator: Join operation | |, enclosed in single quotation marks, representing the string

3. Features

1-third-generation languages

2-Case insensitive

Data storage--sqlite Database storage 2