SQL - Deleting Data and Structure

Commands Covered

• DROP
• ALTER
• DELETE
• WHERE
• UPDATE
• SET

Deleting Databases

Deleting databases is done using the GUI after entering authentication and confirming your selection.

Deleting Tables

Another simple command, use the DROP function to delete a table.

DROP TABLE example_table;

Deleting Columns

First you must specify the table you’re altering, then the row you wish to drop.

ALTER TABLE example_table_2
  DROP COLUMN email;

Deleting Rows

You can delete a single row by referencing a primary key, or a number of rows using logic with the where clause.

Deleting a hypothetical single row:

DELETE FROM example_table
  WHERE 
    emp_no = 99903;

Deleting many hypothetical rows:

DELETE FROM example_table
  WHERE
    start_date <= '2015-01-01'

Deleting Values

If you want to replace a value with null (or another value for that matter), use the UPDATE function. In this example, I replace all zeros with NULL.

UPDATE
  example_table
SET
  value = NULL 
WHERE
  value = 0

That’s all for now!