Relational Database
A relational database is a database composed of multiple two-dimensional tables that can be connected to each other.
advantage:
1. Both use the table structure, the format is consistent, and it is easy to maintain.
2. Use common SQL language to operate, easy to use, and can be used for complex queries.
3. The data is stored in the disk, which is safe.
Table files end in .frm.
SQL statements can be written in one or more lines and end with a semicolon.
not case sensitive.
Comments: Single-line comments -- (space) XXX or (#XXXX)
Multi-line comments: /* xxxxx*/
DDL: Manipulating databases, tables, etc. （Data Definition Language）
DML: Add, delete, and modify table data. (Manipulation)
DQL: query the data in the table. (Query)
DCL: Perform permission control on the database. (Control)