The DELETE statement deletes rows from a table.
DROP COLUMN.Required Privileges
The user must have the DELETE and SELECT privileges on the table.
Synopsis
Parameters
| Parameter | Description |
|---|---|
relation_expr |
The name of the table you want to delete rows from. Deleting from multiple tables in a single statement is not supported. |
AS name |
Create an alias for the table name, completely hiding its original name. All subsequent references to the table must use its alias. Aliases are primarily used with JOIN, which is not yet supported but is coming in a future release. |
WHERE a_expr |
a_expr must be an expression that returns Boolean values using columns (e.g., <column> = <value>). Delete rows that return TRUE.Without a WHERE clause in your statement, DELETE removes all rows from the table. |
RETURNING ... |
Retrieve a table of deleted rows using all columns (*) or specific columns (named in a_expr).To return nothing in the response, not even the number of rows affected, use RETURNING NOTHING. |
AS col_label |
In the retrieved table, change the column label from a_expr to col_label.You can also change column labels with an identifier, but must follow these rules. |
Success Responses
Successful DELETE statements return one of the following:
| Response | Description |
|---|---|
DELETE int |
int rows were deleted.DELETE statements that do not delete any rows respond with DELETE 0. When RETURNING NOTHING is used, this information is not included in the response. |
| Retrieved table | Including the RETURNING clause retrieves the deleted rows, using the columns identified by the clause's parameters.See an example. |
Disk Space Usage After Deletes
Deleting a row does not immediately free up the disk space. This is due to the fact that CockroachDB retains the ability to query tables historically.
If disk usage is a concern, there are two potential solutions. The
first is to reduce the time-to-live
(TTL) for the zone, which will cause garbage collection to clean up
deleted rows more frequently. Second, unlike DELETE,
truncate immediately deletes the entire table, so
consider if you can use TRUNCATE instead.
Select Performance on Deleted Rows
Queries that scan across tables that have lots of deleted rows will have to scan over deletions that have not yet been garbage collected. Certain database usage patterns that frequently scan over and delete lots of rows will want to reduce the time-to-live values to clean up deleted rows more frequently.
Examples
Delete All Rows
You can delete all rows from a table by not including a WHERE clause in your DELETE statement.
> DELETE FROM account_details;
DELETE 7
This is roughly equivalent to TRUNCATE.
> TRUNCATE account_details;
TRUNCATE
As you can see, one difference is that TRUNCATE does not return the number of rows it deleted.
TRUNCATE statement removes all rows from a table by dropping the table and recreating a new table with the same name. This is much more performant than deleting each of the rows.Delete Specific Rows
When deleting specific rows from a table, the most important decision you make is which columns to use in your WHERE clause. When making that choice, consider the potential impact of using columns with the Primary Key/Unique constraints (both of which enforce uniqueness) versus those that are not unique.
Delete Rows Using Primary Key/Unique Columns
Using columns with the Primary Key or Unique constraints to delete rows ensures your statement is unambiguous—no two rows contain the same column value, so it's less likely to delete data unintentionally.
In this example, account_id is our primary key and we want to delete the row where it equals 1. Because we're positive no other rows have that value in the account_id column, there's no risk of accidentally removing another row.
> DELETE FROM account_details WHERE account_id = 1 RETURNING *;
+------------+---------+--------------+
| account_id | balance | account_type |
+------------+---------+--------------+
| 1 | 32000 | Savings |
+------------+---------+--------------+
Delete Rows Using Non-Unique Columns
Deleting rows using non-unique columns removes every row that returns TRUE for the WHERE clause's a_expr. This can easily result in deleting data you didn't intend to.
> DELETE FROM account_details WHERE balance = 30000 RETURNING *;
+------------+---------+--------------+
| account_id | balance | account_type |
+------------+---------+--------------+
| 2 | 30000 | Checking |
| 3 | 30000 | Savings |
+------------+---------+--------------+
The example statement deleted two rows, which might be unexpected.
Return Deleted Rows
To see which rows your statement deleted, include the RETURNING clause to retrieve them using the columns you specify.
Use All Columns
By specifying *, you retrieve all columns of the delete rows.
> DELETE FROM account_details WHERE balance < 23000 RETURNING *;
+------------+---------+--------------+
| account_id | balance | account_type |
+------------+---------+--------------+
| 4 | 22000 | Savings |
+------------+---------+--------------+
Use Specific Columns
To retrieve specific columns, name them in the RETURNING clause.
> DELETE FROM account_details WHERE account_id = 5 RETURNING account_id, account_type;
+------------+--------------+
| account_id | account_type |
+------------+--------------+
| 5 | Checking |
+------------+--------------+
Change Column Labels
When RETURNING specific columns, you can change their labels using AS.
> DELETE FROM account_details WHERE balance < 22500 RETURNING account_id, balance AS final_balance;
+------------+---------------+
| account_id | final_balance |
+------------+---------------+
| 6 | 23500 |
+------------+---------------+
See Also
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