Table of contents
What is DDL
The full English name of DDL is Data Definition Language, which is used to define database objects (databases, tables).
Database operations
Inquire
Query all databases: show databases;
Query the current database: select database();
use
Use database: use database name;
create
Create database: create database if not exists database name;
delete
Delete database: drop database if exists database name;
Database in the above syntax can also be replaced by schema. For example: show schemas;
Table operations
create
create table table name(
Field 1 field type constraint comment field 1 comment,
······
Field n field type constraint comment field n comment
) comment table comment;
Field constraints
| constraint | describe | Keywords |
| non-null constraint | Limit the value of this field to not be null | not null |
| unique constraint | Ensure that all data in the fields are unique and non-duplicate | unique |
| primary key constraints | The primary key is the unique identifier of a row of data and must be non-empty and unique. | primary key |
| Default constraints | When saving data, if the field value is not specified, the default value is used | default |
| foreign key constraints | Let the data of the two tables be connected to ensure the consistency and integrity of the data | foreign key |
type of data
Numeric type
Among thembigint and int are the most commonly used and can be used without them. Thinking too hard about the range of numeric types. For example, the age can be expressed using the int type, and there is no need to deliberately use the tinyint type. The primary key is generally generated using automatic increment, and the type is bigint.
| type | Size (byte) | describe |
| tinyint | 1 | small integer value |
| smallint | 2 | large integer value |
| mediumint | 3 | large integer value |
| int | 4 | large integer value |
| bigint | 8 | extremely large integer value |
| float | 4 | single precision floating point value |
| double | 8 | Double precision floating point value |
| decimal | Decimal values (higher precision) |
string type
char(n), can store up to n characters, less than n characters, occupy n character space. High performance, waste of space.
varchar(n), can store up to n characters, less than n characters, occupying the actual length character space. Low performance, saving space.
| type | Size (bytes) | describe |
| char | 0 - 255 bytes | Fixed length string |
| varchar | 0 - 65535 bytes | variable length string |
| tinyblob | 0 - 255 bytes | Binary data of no more than 255 characters |
| tinytext | 0 - 255 bytes | short text string |
| blob | 0 - 65535 bytes | Long text data in binary form |
| text | 0 - 65535 bytes | long text data |
| mediumblob | 0 - 16777215 bytes | Medium-length text data in binary form |
| mediumtext | 0 - 16777215 bytes | Medium length text data |
| lungblob | 0 - 4294967295 bytes | Very large text data in binary form |
| longtext | 0 - 4294967295 bytes | Very large text data |
datetime type
The field create_time creation time and the field update_time update time both use datetime type.
| type | Format | describe |
| date | YYYY - MM - DD | date value |
| time | HH : MM : SS | time value or duration |
| year | YYYY | year value |
| datetime | YYYY - MM - DD HH : MM : SS | Mix date and time values |
| timestamp | YYYY - MM - DD HH : MM : SS | Mixed date and time values, timestamps |
Inquire
Query all tables in the current database: show tables;
Query table structure: desc table name;
Query the table creation statement: show create table table name;
Revise
Add fields: alter table table name add field name type comment comment constraint;
Modify field type: alter table table name modify field name new data type;
Modify field names and field types: alter table table name change old field name new field name type comment comment constraint;
Delete a field: alter table table name drop column field name;
Modify the table name: rename table table name to new table name;
delete
Delete table: drop table if exists table name;