SQL Notes (2) - MySQL Table Operations and Indexes (Favorite Edition)

This article records in detail how to modify the MySQL table structure through commands, such as adding columns, deleting columns, etc.; not only have you learned, you have also learned, and you can collect ashes~

before the start

The previous article created some tables, and the ER diagram is as follows. This article scoreoperates on the table, the scene is to add a new 备注remarksfield, the data type is varchar, the length 1000is allowed null, and the default value is 无;

View table structure

To view the detailed design information of a specific table in the MySQL database, you can use DESCRIBEthe command, which can display information such as the name, data type, default value, null constraint, and key constraint of all columns in the table. For example:

View the structure of the score table

mysql> describe score;
+------------+--------+------+-----+---------+----------------+
| Field      | Type   | Null | Key | Default | Extra          |
+------------+--------+------+-----+---------+----------------+
| id         | int    | NO   | PRI | NULL    | auto_increment |
| student_id | int    | NO   | MUL | NULL    |                |
| course_id  | int    | NO   | MUL | NULL    |                |
| score      | double | NO   |     | NULL    |                |
+------------+--------+------+-----+---------+----------------+
4 rows in set (0.01 sec)

Enter the command describe score;to display scoreall the fields of the table, as well as the corresponding name, data type, default value, null value constraint, key constraint and other information of each field;

MUL here Keyis mainly because these two fields have foreign key constraints, which are constrained to correspond to the corresponding fields of another table;

View the structure of a column of a table

mysql> describe score id;
+-------+------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra          |
+-------+------+------+-----+---------+----------------+
| id    | int  | NO   | PRI | NULL    | auto_increment |
+-------+------+------+-----+---------+----------------+
1 row in set (0.00 sec)

Enter describe score id;to view scorethe structural information of the id column of the table; the usage isdescribe 表名列名;

Here you can also use show columns to view the structure :

View a table structure:SHOW COLUMNS FROM table_name；

View the structure of a field in a table:SHOW COLUMNS FROM table_name LIKE 'column_name';

add column

Following the above scenario, add a new 备注remarksfield to the score table, the data type is varchar, the length 1000is allowed null, and the default value is 无;

mysql> ALTER TABLE score ADD remarks VARCHAR(1000) NULL DEFAULT '无';
Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0

The above command completes the insertion of the data column; let's look at the table structure after adding the column:

mysql> describe score;
+------------+---------------+------+-----+---------+----------------+
| Field      | Type          | Null | Key | Default | Extra          |
+------------+---------------+------+-----+---------+----------------+
| id         | int           | NO   | PRI | NULL    | auto_increment |
| student_id | int           | NO   | MUL | NULL    |                |
| course_id  | int           | NO   | MUL | NULL    |                |
| score      | double        | NO   |     | NULL    |                |
| remarks    | varchar(1000) | YES  |     | 无      |                |
+------------+---------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)

As above, our scenario requirements are completed;

Among them, the new command ALTER TABLEis used to modify the table structure, scorewhich is the name of the table to be modified, ADD remarksindicating that a remarkscolumn named is added, VARCHAR(1000)indicating that the data type is varchar, and the length is 1000, NULLindicating that the column is allowed to be null, and DEFAULT '无'indicating the default value 无.

modify column

Here, the value of modifying the comment column is not allowed to be empty, and the default value 空备注is the scene;

mysql> alter table score modify column remarks varchar(1000) not null default '无备注';
Query OK, 0 rows affected (0.08 sec)
Records: 0  Duplicates: 0  Warnings: 0

Use alter table score modify column remarks varchar(1000) not null default '无备注';it to complete our requirements, the changed structure:

mysql> describe score;
+------------+---------------+------+-----+-----------+----------------+
| Field      | Type          | Null | Key | Default   | Extra          |
+------------+---------------+------+-----+-----------+----------------+
| id         | int           | NO   | PRI | NULL      | auto_increment |
| student_id | int           | NO   | MUL | NULL      |                |
| course_id  | int           | NO   | MUL | NULL      |                |
| score      | double        | NO   |     | NULL      |                |
| remarks    | varchar(1000) | NO   |     | 无备注    |                |
+------------+---------------+------+-----+-----------+----------------+
5 rows in set (0.00 sec)

Command format:ALTER TABLE table_name MODIFY COLUMN column_name column_type NOT NULL DEFAULT '默认值';

table_nameIndicates the name of the table that needs to be modified, column_namethe name of the column that needs to be modified, column_typethe original data type of the column, NOT NULLthe setting of the column as a non-null constraint, DEFAULT '默认值'and the setting of the default value of the column 默认值.

Note : column_typeis required. In SQL, modifying the data type of a column in a table is an important operation, because the data type determines the type and range of data that the column can store. If you do not specify the data type, MySQL cannot parse the statement, and an error occurs:
mysql> alter table score modify column remarks not null default '无备注';
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'not null default '无备注'' at line 1

duplicated column

Here rename remarks to new_remarks;

mysql> alter table score rename column remarks TO new_remarks;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

Effect:

mysql> describe score;
+-------------+---------------+------+-----+-----------+----------------+
| Field       | Type          | Null | Key | Default   | Extra          |
+-------------+---------------+------+-----+-----------+----------------+
| id          | int           | NO   | PRI | NULL      | auto_increment |
| student_id  | int           | NO   | MUL | NULL      |                |
| course_id   | int           | NO   | MUL | NULL      |                |
| score       | double        | NO   |     | NULL      |                |
| new_remarks | varchar(1000) | NO   |     | 无备注    |                |
+-------------+---------------+------+-----+-----------+----------------+
5 rows in set (0.01 sec)

Command usage:ALTER TABLE table_name RENAME COLUMN old_column_name TO new_column_name;

table_nameIndicates the name of the table that needs to be operated, old_column_namethe name of the column that needs to be renamed, new_column_nameand the new column name, and the keyword is RENAME;

change column position

This scenario is very simple, that is, to change the position of the comment column, but due to the reason of SQL syntax, what needs to be distinguished is that the command to move to the back of a certain column or to the beginning of the table is different;

mysql> ALTER TABLE score MODIFY COLUMN new_remarks varchar(1000) AFTER student_id;
Query OK, 0 rows affected (0.09 sec)
Records: 0  Duplicates: 0  Warnings: 0

This is the command after moving to a certain column, the effect is as follows:

mysql> describe score;
+-------------+---------------+------+-----+---------+----------------+
| Field       | Type          | Null | Key | Default | Extra          |
+-------------+---------------+------+-----+---------+----------------+
| id          | int           | NO   | PRI | NULL    | auto_increment |
| student_id  | int           | NO   | MUL | NULL    |                |
| new_remarks | varchar(1000) | YES  |     | NULL    |                |
| course_id   | int           | NO   | MUL | NULL    |                |
| score       | double        | NO   |     | NULL    |                |
+-------------+---------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

Here is the command to move to the header:

mysql> alter table score modify column new_remarks varchar(1000) first;
Query OK, 0 rows affected (0.12 sec)
Records: 0  Duplicates: 0  Warnings: 0

The effect after moving:

mysql> describe score;
+-------------+---------------+------+-----+---------+----------------+
| Field       | Type          | Null | Key | Default | Extra          |
+-------------+---------------+------+-----+---------+----------------+
| new_remarks | varchar(1000) | YES  |     | NULL    |                |
| id          | int           | NO   | PRI | NULL    | auto_increment |
| student_id  | int           | NO   | MUL | NULL    |                |
| course_id   | int           | NO   | MUL | NULL    |                |
| score       | double        | NO   |     | NULL    |                |
+-------------+---------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)

Command after moving to a column:ALTER TABLE table_name MODIFY COLUMN column_name column_definition AFTER other_column_name;

table_nameIndicates the name of the table that needs to be operated, column_nameindicates the name of the column that needs to be changed, and column_definitionindicates the column definition, which can include information such as data type, default value, etc. AFTERThe keyword is followed by which column the column needs to be placed, and other_column_nameit is the column of other columns name.

Move to the very beginning:ALTER TABLE table_name MODIFY COLUMN column_name column_definition FIRST;

table_nameIt is the name of the table to be operated, column_namethe name of the column to be moved, column_definitionand the definition of the column, FIRSTindicating that the column will be moved to the beginning of the table.

Note :

When using ALTER TABLEthe command to modify a column, if the data type is omitted, an error will be reported because a new data type must be specified. That is, the above command cannot omit varchar(1000)fields;

In MySQL, LASTis not a legal keyword and will cause a syntax error. To move a column to the end of the table, use AFTERthe keyword to specify the name of the column to move the column to. That is: alter table score modify column new_remarks varchar(1000) LAST;it is a wrong statement!

delete column

Here we delete the newly created comment field:

mysql> alter table score drop column new_remarks;
Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0

After deleting, the original effect is restored:

mysql> describe score;
+------------+--------+------+-----+---------+----------------+
| Field      | Type   | Null | Key | Default | Extra          |
+------------+--------+------+-----+---------+----------------+
| id         | int    | NO   | PRI | NULL    | auto_increment |
| student_id | int    | NO   | MUL | NULL    |                |
| course_id  | int    | NO   | MUL | NULL    |                |
| score      | double | NO   |     | NULL    |                |
+------------+--------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

We used DROP COLUMNa clause to indicate that the column should be dropped. Note that this permanently removes the column from the table!

Q&A

What does Extra in the table structure mean?

The Extra field in the MySQL table structure is used to display additional information and comments. This field provides some important information, such as self-increment, default value, primary key information, etc. It can tell the user more about the column and help the user better understand the role and purpose of the column.

Common Extra field values include:

auto_increment: Indicates that the column is an auto-increment primary key.

on update CURRENT_TIMESTAMP: Indicates that the column is set to automatically update the timestamp function.

DEFAULT 'xxx': Indicates that the column has a default value of xxx.

What does the key of the table structure represent?

PRI: Indicates that the column is the primary key, which is a special index used to uniquely identify each row of data in the table. Each table can have at most one primary key, and the data in the primary key restriction table cannot be duplicated and cannot be NULL.

MUL: Non-unique index, when the index includes multiple duplicate key values, the Key attribute value will be displayed as mul.

UNI: Indicates that the column is a unique key. A unique key can also be used to uniquely identify each row of data in a table, but it allows NULL values. A table can have multiple unique keys.

FULLTEXT: Indicates that the column is a full-text index. Full-text indexing is used to optimize full-text searches, such as keyword searches on article titles and text. A table can have at most one full-text index.

`` (null): Indicates that the column is neither a primary key nor a unique key.

For example, when DESCRIBEviewing a studentstable named with the command, you might get the following output:
mysql> DESCRIBE students;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id       | int(11)     | NO   | PRI | NULL    |       |
| name     | varchar(50) | YES  |     | NULL    |       |
| gender   | char(1)     | YES  |     | NULL    |       |
| age      | int(11)     | YES  |     | NULL    |       |
| email    | varchar(80) | YES  | UNI | NULL    |       |
| address  | varchar(100)| YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
Among them, idthe column Keyis PRI, indicating that it is a primary key; emailthe column Keyis UNI, indicating that it is a unique key; the other columns Keyare ""(empty), indicating that they are neither primary keys nor unique keys.

what is index

A MySQL index is a data structure that helps MySQL quickly obtain data, just like the catalog of a book can quickly access specific information. It can improve the speed of database queries. The basic principle is to trade space for time, and store indexes in files on disk to speed up reading and writing. Indexes allow you to quickly access specific information in a table by sorting the values of one or more columns in the table. The realization of the index depends on the design of the storage engine and the data retrieval engine of the database, so choosing an appropriate data structure can improve the performance of data retrieval, and for a database with large-scale data storage, it is very important to select an appropriate and efficient search algorithm.

Primary key index: The values in the indexed column must be unique, and null values are not allowed.

Ordinary index: The basic index type in MySQL, there are no restrictions, and it is allowed to insert duplicate values and null values in the columns defining the index.

Unique index: The values in the indexed column must be unique, but null values are allowed.

Full-text index: Full-text indexes can only be created on fields of text type CHAR, VARCHAR, and TEXT. When the field length is relatively large, if you create an ordinary index, the efficiency is relatively low when performing like fuzzy queries. At this time, you can create a full-text index. Full-text indexes are available in both MyISAM and InnoDB.

Spatial index: MySQL version after 5.7 supports spatial index, and supports OpenGIS geometric data model. MySQL follows the OpenGIS geometric data model rules for spatial indexes.

Prefix index: When creating an index on a text type column such as CHAR, VARCHAR, and TEXT, the length of the index column can be specified, but the value type cannot be specified.

Others (classified by the number of index columns)

single column index

Composite index: The use of composite index needs to follow the leftmost prefix matching principle (leftmost matching principle). Generally, composite indexes are used instead of multiple single-column indexes when conditions permit.

How to view the indexes in a table

SHOW INDEX FROM score;
Here is an explanation of these fields:

Table: The name of the table where the index resides.

Non_unique: Whether the index allows duplicate values, 0 indicates a unique index, and 1 indicates that duplicate values are allowed.

Key_name: The name of the index.

Seq_in_index: The order of the column in the index, starting from 1.

Column_name: The column name included in the index.

Collation: The character set collation to use for this column, or NULL if the column is not in any index.

Cardinality: An estimate of the number of unique values in the index, not necessarily accurate.

Sub_part: If it is a prefix index, it indicates the length of the indexed string, otherwise it is NULL.

Packed: If it is a PACK_KEYS index type, it indicates the compression rate used, otherwise it is NULL.

Null: Whether the column allows NULL values, if allowed, it is YES, otherwise it is NO.

Index_type: Index type, such as Btree, Hash, etc.

Comment: A comment for the index.

Index_comment: The character set and collation of index comments.

Visible: Whether the index is visible, 0 means hidden, 1 means visible.

Expression: The expression or function name if this index is the result of a function or expression, otherwise NULL.

How to view the constraints of a table

mysql> SELECT CONSTRAINT_NAME, CONSTRAINT_TYPE
    -> FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS
    -> WHERE TABLE_SCHEMA = 'student_score_db' and table_name='score';
+------------------+-----------------+
| CONSTRAINT_NAME  | CONSTRAINT_TYPE |
+------------------+-----------------+
| PRIMARY          | PRIMARY KEY     |
| fk_score_course  | FOREIGN KEY     |
| fk_score_student | FOREIGN KEY     |
+------------------+-----------------+
3 rows in set (0.00 sec)
The above SQL statement will return all constraint names and types for the given table, such as PRIMARY KEY、FOREIGN KEY、UNIQUEetc. If you want to see the details of a specific constraint (such as the columns included), you can use INFORMATION_SCHEMA.KEY_COLUMN_USAGEthe table to query.

Original link: https://blog.jiumoz.com/archives/sql-bi-ji-2mysql-de-biao-cao-zuo-yu-suo-yin