# SQL beginner's Guide # Learning Notes (1)

The Tip1:SQL sentence should be logically analogous to English (adverbial post). Is a language that only focuses on relational databases\ n

DML (Data Manipution Language) data manipulation language, that is, operations on data; DDL (Data Definition Language) data definition language, that is, operations on the database itself; and DCL (Data Control Language) data control language, that is, maintaining the security of the database.

In SQL, records (record) and fields (field) correspond to the concepts of rows and columns.

Primary key: identification and uniqueness, cannot contain NULL; foreign keys: direct reference from one column in one table to another, relationship between child table and parent table\ n

For data types, the following

General description sql_servermysqloracle

Bit

Bitbit (none) integer (no decimal point) intintnumberdecimal (with decimal point) decimaldecimalnumberrealfloatfloatnumber General description sql_servermysqloracle variable length varcharvarcharvarchar2 fixed length charCharchar General description sql_servermysqloracle date Datedate (none) date and time Datetimedatetimedate

Spaces and null values (NULL) are different. Null value