先决条件 – DDL、DQL、DML、DCL 和 TCL 命令
DDL：
DDL 是数据定义语言，用于定义数据结构。例如：create table、alter table 是 SQL 中的指令。
DML：
DML 是数据操作语言，用于操作数据本身。例如：插入、更新、删除是 SQL 中的指令。
DDL和DML的区别：
| DDL | DML |
|---|---|
| It stands for Data Definition Language. | It stands for Data Manipulation Language. |
| It is used to create database schema and can be used to define some constraints as well. | It is used to add, retrieve or update the data. |
| It basically defines the column (Attributes) of the table. | It add or update the row of the table. These rows are called as tuple. |
| It doesn’t have any further classification. | It is further classified into Procedural and Non-Procedural DML. |
| Basic command present in DDL are CREATE, DROP, RENAME, ALTER etc. | BASIC command present in DML are UPDATE, INSERT, MERGE etc. |
| DDL does not use WHERE clause in its statement. | While DML uses WHERE clause in its statement. |