1. Database operations
There is no case sensitivity in the database! ! !
1.1 Display database
show databases ;
As shown in the picture:
1.2 Create database
create database [if not exists] database name;
As shown in the picture:
1.3 Using database
use database name;
As shown in the picture:
This operation is used to operate tables in the library. We will not talk about it now. It will be used in the next article when we talk about adding, deleting, checking, and modifying tables . Whenever you operate a table, you must add this sentence before using the table.
1.4 Delete library
drop database [ if exists ] database name;
As shown in the picture:
2. Common data types
2.1 Integer and floating point types
| type of data | size | illustrate | Corresponding JAVA type |
| bit[(M)] | M represents the specified number of digits, the default is 1, you can not add it | Binary number, M ∈[1,64] Storage value range [0,2^M-1] |
boolean |
| tinyint | 1 byte | Byte | |
| smallint | 2 bytes | Short | |
| int | 4 bytes | Integer | |
| bigint | 8 bytes | Long | |
| float(M,D) | 4 bytes | Single precision, M specifies the length, D specifies the number of decimal places, A loss of precision will occur |
Float |
| double(M,D) | 8 bytes | Double | |
| decimal(M,D） | M/D maximum value +2 | Double precision, M specifies the length, D specifies the number of decimal places, Exact value |
DigDecimal |
| numeric(M,D) | M/D maximum value +2 | Double precision, M specifies the length, D specifies the number of decimal places, Exact value |
BigDecimal |
Note: There is an unsigned type in mysql, but its use is not recommended and may cause data errors.
2.2 String type
| type of data | size | illustrate | Corresponding JAVA type |
| varchar(size) | 0-65535 bytes | size indicates how many characters there are (a Chinese character is also a character) | String |
| test | 0-65535 bytes | long text data | String |
| mediumtext | 0-16777215 bytes | Medium and long text data | String |
| blob | 0-65535 bytes | binary text data | byte[] |
2.3 Date type
| type of data | size | illustrate | Corresponding JAVA type |
| datetime | 8 bytes | The range is from 1000 to 9999. Time zone retrieval and conversion will not be performed. | java.util.Date java.sql.Timestamp |
| timestamp | 4 bytes | Ranges from 1970 to 2023, automatically retrieves current time zone and converts | java.util.Date java.sql.Timestamp |
3. Table operations
3.1 View table structure
desc table name;
3.2 Create table
create table table name(field name type, field name type, ...);
Note: Before using the table, be sure to write use database name;
3.3 Delete table
drop table [ if exists ] 表名 ;
