Note
/* 多行注释 */
# 单行注释
-- 单行注释(注意：两个但横线后面必须加空格)type of data
1. Plastic
tinyint, smallint, mediumint, int, bigint
small integer large integer large integer large integer greatly integer
2. Float
float、double
3. decimal type
decimal (common: to ensure that data is not lost)
as: 100.32 ----- "decimal (5,2)
4. Character
1.char
2.varchar
3.text
4.blob
5.enum enumeration (one of many)
6.set (multiple choice and more)
5. Date and Time
1.year
2. DATE
3.time
4.datetime (not 5.5)
5.timestamp timestamp
A null value may be used as the = assignment as a condition for determination is null
Non-empty is not null condition is determined non-empty