Synopsis

UPDATE [ONLY] table [[AS] alias]
   SET {column = {expression | DEFAULT} |
   (column [, ...]) = ({expression | DEFAULT} [, ...])} [, ...]
   [FROM fromlist]
   [WHERE condition | WHERE CURRENT OF cursor_name ]

Description

UPDATE changes the values of the specified columns in all rows that satisfy the condition. Only the columns to be modified need be mentioned in the SET clause; columns not explicitly modified retain their previous values.

By default, UPDATE will update rows in the specified table and all its subtables. If you wish to only update the specific table mentioned, you must use the ONLY clause.

There are two ways to modify a table using information contained in other tables in the database: using sub-selects, or specifying additional tables in the FROM clause. Which technique is more appropriate depends on the specific circumstances.

If the WHERE CURRENT OF clause is specified, the row that is updated is the one most recently fetched from the specified cursor.

You must have the UPDATE privilege on the table to update it, as well as the SELECT privilege to any table whose values are read in the expressions or condition.

UPDATE count

Parameters

ONLY

If specified, update rows from the named table only. When not specified, any tables inheriting from the named table are also processed.

table

The name (optionally schema-qualified) of an existing table.

alias

A substitute name for the target table. When an alias is provided, it completely hides the actual name of the table. For example, given UPDATE foo AS f, the remainder of the UPDATE statement must refer to this table as f not foo.

column

The name of a column in table. The column name can be qualified with a subfield name or array subscript, if needed. Do not include the table's name in the specification of a target column.

expression

An expression to assign to the column. The expression may use the old values of this and other columns in the table.

DEFAULT

Set the column to its default value (which will be NULL if no specific default expression has been assigned to it).

fromlist

A list of table expressions, allowing columns from other tables to appear in the WHERE condition and the update expressions. This is similar to the list of tables that can be specified in the FROM clause of a SELECT statement. Note that the target table must not appear in the fromlist, unless you intend a self-join (in which case it must appear with an alias in the fromlist).

condition

An expression that returns a value of type boolean. Only rows for which this expression returns true will be updated.

cursor_name

The name of the cursor to use in a WHERE CURRENT OF condition. The row to be updated is the one most recently fetched from the cursor. The cursor must be a simple (non-join, non-aggregate) query on the UPDATE command target table. See DECLARE for more information about creating cursors.

WHERE CURRENT OF cannot be specified together with a Boolean condition.

output_expression

An expression to be computed and returned by the UPDATE command after each row is updated. The expression may use any column names of the table or table(s) listed in FROM. Write * to return all columns.

output_name

A name to use for a returned column.

Notes

SET is not allowed on the Greenplum distribution key columns of a table.

When a FROM clause is present, what essentially happens is that the target table is joined to the tables mentioned in the from list, and each output row of the join represents an update operation for the target table. When using FROM you should ensure that the join produces at most one output row for each row to be modified. In other words, a target row should not join to more than one row from the other table(s). If it does, then only one of the join rows will be used to update the target row, but which one will be used is not readily predictable.

Because of this indeterminacy, referencing other tables only within sub-selects is safer, though often harder to read and slower than using a join.

Execution of UPDATE and DELETE commands directly on a specific partition (child table) of a partitioned table is not supported. Instead, these commands must be executed on the root partitioned table, the table created with the CREATE TABLE command.

Examples

Change the word Drama to Dramatic in the column kind of the table films:

UPDATE films SET kind = 'Dramatic' WHERE kind = 'Drama';

Adjust temperature entries and reset precipitation to its default value in one row of the table weather:

UPDATE weather SET temp_lo = temp_lo+1, temp_hi = 
temp_lo+15, prcp = DEFAULT
WHERE city = 'San Francisco' AND date = '2006-07-03';

Use the alternative column-list syntax to do the same update:

UPDATE weather SET (temp_lo, temp_hi, prcp) = (temp_lo+1, 
temp_lo+15, DEFAULT)
WHERE city = 'San Francisco' AND date = '2006-07-03';

Increment the sales count of the salesperson who manages the account for Acme Corporation, using the FROM clause syntax (assuming both tables being joined are distributed in Greenplum Database on the id column):

UPDATE employees SET sales_count = sales_count + 1 FROM 
accounts
WHERE accounts.name = 'Acme Corporation'
AND employees.id = accounts.id;

Perform the same operation, using a sub-select in the WHERE clause:

UPDATE employees SET sales_count = sales_count + 1 WHERE id =
  (SELECT id FROM accounts WHERE name = 'Acme Corporation');

Attempt to insert a new stock item along with the quantity of stock. If the item already exists, instead update the stock count of the existing item. To do this without failing the entire transaction, use savepoints.

BEGIN;
-- other operations
SAVEPOINT sp1;
INSERT INTO wines VALUES('Chateau Lafite 2003', '24');
-- Assume the above fails because of a unique key violation,
-- so now we issue these commands:
ROLLBACK TO sp1;
UPDATE wines SET stock = stock + 24 WHERE winename = 'Chateau 
Lafite 2003';
-- continue with other operations, and eventually
COMMIT;

Compatibility

This command conforms to the SQL standard, except that the FROM clause is a Greenplum Database extension.

According to the standard, the column-list syntax should allow a list of columns to be assigned from a single row-valued expression, such as a sub-select:

UPDATE accounts SET (contact_last_name, contact_first_name) =
    (SELECT last_name, first_name FROM salesmen
     WHERE salesmen.id = accounts.sales_id);

This is not currently implemented — the source must be a list of independent expressions.

Some other database systems offer a FROM option in which the target table is supposed to be listed again within FROM. That is not how Greenplum Database interprets FROM. Be careful when porting applications that use this extension.

See Also

DECLARE, DELETE, SELECT, INSERT