SQL Delete | Delete from Statement - sql - sql tutorial - learn sql
- The WHERE clause specifies which record(s) that should be deleted. If you omit the WHERE clause, all records in the table will be deleted!
- The DELETE FROM statement in SQL is used to remove records from a table.
- DELETE FROM command cannot delete any rows of data that would violate
Syntax
- The syntax for the DELETE FROM statement is as follows:
DELETE FROM "table_name"
WHERE "condition";
- The WHERE clause is important here.
- Without specifying a condition, all records from the table will be deleted.
- "Condition" can be simple (such as "Sales > 500") or complex (such as from the result of a subquery).
Tags : sql tutorial , pl sql tutorial , mysql tutorial , oracle tutorial , learn sql , sql server tutorialExamples
Examples
- Two examples of how to use the DELETE FROM statement are shown below.
Example 1: DELETE FROM using a simple condition
- We use the following table as the starting point.
- Table Store_Information
| Store_Name | Sales | Txn_Date |
|---|---|---|
| Los Angeles | 1500 | Jan-05-1999 |
| San Diego | 250 | Jan-07-1999 |
| Los Angeles | 300 | Jan-08-1999 |
| Boston | 700 | Jan-08-1999 |
- We decide not to keep any information on Los Angeles in this table. To accomplish this, we type the following SQL:
Ø DELETE FROM Store_Information
WHERE Store_Name = 'Los Angeles';
- Now the table becomes,
Table Store_Information
| Store_Name | Sales | Txn_Date |
|---|---|---|
| San Diego | 250 | Jan-07-1999 |
| Boston | 700 | Jan-08-1999 |
Example 2: DELETE FROM using the results from a subquery
- In Example 1, the criteria we use to determine which rows to delete is quite simple. We can also use a more complex condition. Below is an example where we use as the condition. Assume we have the following two tables:
- Table Store_Information
| Store_Name | Sales | Txn_Date |
|---|---|---|
| Los Angeles | 1500 | Jan-05-1999 |
| San Diego | 250 | Jan-07-1999 |
| Los Angeles | 300 | Jan-08-1999 |
| Boston | 700 | Jan-08-1999 |
- Table Geography
| Region_Name | Store_Name |
|---|---|
| East | Boston |
| East | New York |
| West | Los Angeles |
| West | San Diego |
| Store_Name | Sales | Txn_Date |
|---|---|---|
| Los Angeles | 1500 | Jan-05-1999 |
| San Diego | 250 | Jan-07-1999 |
| Los Angeles | 300 | Jan-08-1999 |
- We want to remove data for all stores in the East region from Store_Information (assuming that a store is either in the East region or the West region-it cannot be in more than one region). We use the following SQL statement to accomplish this:
DELETE FROM Store_Information
WHERE Store_Name IN
(SELECT Store_Name FROM Geography
WHERE Region_Name = 'East');
- Upon execution, the Store_Information table becomes,
- If we leave out the clause in a DELETE FROM command, we will delete all rows from the table. Most times, this is not what we intend to do. To prevent this, it is a best practice in database management to always run the corresponding statement first to make sure the rows selected are the ones we intend to remove from the table. This can be done by replacing "DELETE" with "SELECT *".
Exercises
- For the questions below, we use the following table as the starting point:
Table Clients
| Customer_ID | Last_Name | First_Name | City | State | Join_Date |
|---|---|---|---|---|---|
| 2 | Larry | Larry | Seattle | WA | Oct-15-2012 |
| 5 | Aaron | Wallace | Denver | CO | Oct-18-2012 |
| 6 | Jayson | Fortran | Raleigh | NC | Oct-24- 2012 |
| 12 | jill | Dobbs | Buffalo | NY | Nov-15-2012 |
| 13 | Lisa | Yamaguchi | San Diego | CA | Nov-15- 2012 |
| 20 | Ally | Smith | Seattle | WA | Nov-25-2012 |
| 67 | Teyu | Lee | Cupertino | CA | Jan-11-2013 |
Which of the following SQL statements is valid? (There may be more than one answer)
a) DELETE * FROM Clients WHERE State = 'CO';
b) DELETE FROM Clients WHERE State = 'CO';
c) DELETE FROM Clients HAVING State = 'CO';
d) DELETE FROM Clients WHERE Customer_ID < 10;
- How many rows are deleted after the following SQL statement is executed?
- DELETE FROM Clients WHERE State = 'CO';
Answer
1 row- What is the effect of the following SQL?
- DELETE FROM Clients WHERE 1 = 1;
Answer
All rows are deleted from the table. This is because the condition
in the WHERE clause is true for all rows.sql tutorial , pl sql tutorial , mysql tutorial , oracle tutorial , learn sql , sql server tutorialDelete vs Truncate
Delete vs Truncate
