How to delete big data in sql

The best way to delete big data depends on the specific situation. Direct deletion methods include TRUNCATE TABLE and DELETE FROM TABLE WHERE, while batch deletion methods include DELETE ... IN (SELECT ...) and using cursors. Other methods include partition deletion, foreign key constraints, and truncation and recreation. The specific choice depends on the table size, the complexity of the deletion condition, the recovery data requirement and acceptable processing time.

SQL method to delete big data

Direct deletion method:

• TRUNCATE TABLE: Use the TRUNCATE TABLE command to quickly delete all data in a table, including foreign keys and indexes. However, this command cannot be rolled back, so it is not suitable for scenarios where data needs to be restored.
• DELETE FROM TABLE WHERE: Use the DELETE FROM command plus the WHERE clause to delete data that meets certain conditions. However, for large tables, this method can be very slow.

Batch deletion method:

• DELETE ... IN (SELECT ...): Using this method, multiple rows matching the data returned by the subquery can be deleted in a single query. For example:

 <code class="sql">DELETE FROM table WHERE id IN (SELECT id FROM table2 WHERE condition);</code>

• Use cursors: Cursors allow row by row and delete rows that meet certain criteria. This method is slower than direct deletion, but allows for finer control of the data. For example:

 <code class="sql">DECLARE cursor_name CURSOR FOR SELECT id FROM table WHERE condition; OPEN cursor_name; FETCH cursor_name INTO variable; WHILE variable IS NOT NULL DO DELETE FROM table WHERE id = variable; FETCH cursor_name INTO variable; END WHILE; CLOSE cursor_name;</code>

Other methods:

• Partition Deletion: For partitioned tables, you can delete the entire partition to quickly delete data.
• Foreign Key Constraint: If the table has foreign key constraints with other tables, deleting records in the parent table will automatically delete relevant records in the child table.
• Truncate and recreate: For very large tables, consider truncating the table and recreating it. This will be faster than deleting directly, but will lose index and foreign keys.

Choose the best method:

Choosing the best way to delete big data depends on the following factors:

• Table size and complexity
• Complexity of deletion conditions
• Is it necessary to recover data
• Acceptable processing time

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