Also known as field attribute column properties, a total of six attributes in mysql: null, a default value, column descriptions, primary keys, automatic keys and unique growth
1, Null property
NULL attribute : Represents field empty
If the corresponding value of YES indicates that the field may be NULL
note:
(1) In the design of the table, try not to let the data is empty
(2) Mysql record length of 65,535 bytes, if a table has a field allowed is NULL, then the system will be designed that retain a memory byte NULL, the final effective length of 65,534 bytes is stored.
2, the default value
Default: default value , when the field is designed, if the default conditions permit, the user does not insert the data, then the data may be used to fill the prepared: Generally filled with NULL
Test: insert data - no data value is provided to a current field
Default another layer using keywords: display informing field default value: performing data when inserted, directly use the default value for the field
3 , column describes
Column Description: comment, it is a dedicated maintenance explanatory notes to developers
| The basic syntax: comment 'Field Description'; |
View Comment: Statement by looking at the table must be created
