MySQL - Delete Query - Remove Records

In MySQL, the DELETE statement is used to remove one or more rows from a table based on certain conditions. Here's the basic syntax for a DELETE query in MySQL:

DELETE FROM table_name WHERE condition;

Here are some examples of how to use the DELETE statement in MySQL:

Delete all rows from a table:

DELETE FROM my_table;

This will delete all rows from the my_table table.

Delete rows that meet a certain condition:

DELETE FROM my_table WHERE id = 1;

This will delete all rows from the my_table table where the id column is equal to 1.

Delete rows using multiple conditions:

DELETE FROM my_table WHERE name = 'John' AND age > 30;

This will delete all rows from the my_table table where the name column is equal to 'John' and the age column is greater than 30.

Delete rows using a subquery:

DELETE FROM my_table WHERE id IN (SELECT id FROM another_table WHERE condition);

This will delete all rows from the my_table table where the id column is found in the results of the subquery.

The TRUNCATE statement is used to delete all rows from a table, but unlike the DELETE statement, it does not log the individual row deletions. Instead, TRUNCATE simply deallocates the data pages associated with the table, and resets the table to its original state (i.e., empty).

Here's the basic syntax for a TRUNCATE statement in MySQL:

TRUNCATE TABLE table_name;

Here are some examples of how to use the Truncate statement in MySQL:

Truncate a table:

TRUNCATE TABLE my_table;

This will remove all data from the my_table table.

Truncate multiple tables:

TRUNCATE TABLE my_table1, my_table2, my_table3;

This will remove all data from three tables: my_table1, my_table2, and my_table3.

Truncate a table with foreign key constraints:

If my_table1 has foreign key constraints on other tables, you may need to temporarily disable the constraints before truncating the table:

SET FOREIGN_KEY_CHECKS=0;

TRUNCATE TABLE my_table1;

SET FOREIGN_KEY_CHECKS=1;

This will disable foreign key checks, truncate the my_table1 table, and then re-enable foreign key checks.

TRUNCATE and DELETE are both SQL statements used to remove data from a table in MySQL, but they work in different ways and have different effects on the table and the database.

Here are some of the main differences between TRUNCATE and DELETE in MySQL:

• Logging: When you use DELETE, each row that is deleted is logged in the database's transaction log. This can cause the log to become quite large if you are deleting many rows. In contrast, TRUNCATE does not log each individual row deletion. Instead, it simply deallocates the data pages associated with the table.
• Speed: TRUNCATE is generally faster than DELETE, especially for large tables, because it does not log each row deletion. This can make it a better choice if you need to quickly remove all data from a table.
• Rollback: Unlike DELETE, TRUNCATE cannot be rolled back. Once you truncate a table, the data is gone for good.
• Table structure: TRUNCATE removes all rows from a table and resets the table to its original structure, whereas DELETE only removes rows from the table and does not affect the structure of the table.
• Triggers: DELETE can trigger delete triggers that may be associated with the table, whereas TRUNCATE does not trigger any triggers.
• Foreign key constraints: TRUNCATE resets the auto-increment counter of a table, while DELETE doesn't. Also, if a table has foreign key constraints on other tables, TRUNCATE may not work if there are dependencies between the tables, whereas DELETE can still work if it satisfies the foreign key constraints.

In summary, TRUNCATE is a faster and more efficient way to remove all data from a table, but it cannot be rolled back and does not trigger any triggers associated with the table. DELETE is a more flexible option that allows you to remove individual rows from a table, but it is slower and can fill up the transaction log if you are deleting many rows.