| PostgreSQL 8.1.23 Documentation | ||||
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UPDATE [ ONLY ] table SET column = { expression | DEFAULT } [, ...]
[ FROM fromlist ]
[ WHERE condition ]
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.
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.
The name (optionally schema-qualified) of the table to update.
The name of a column in table. The column name can be qualified with a subfield name or array subscript, if needed.
An expression to assign to the column. The expression may use the old values of this and other columns in the table.
Set the column to its default value (which will be NULL if no specific default expression has been assigned to it).
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).
An expression that returns a value of type boolean. Only rows for which this expression returns true will be updated.
On successful completion, an UPDATE command returns a command tag of the form
UPDATE count
The count is the number of rows updated. If count is 0, no rows matched the condition (this is not considered an error).
When a FROM clause is present, what essentially happens is that the target table is joined to the tables mentioned in the fromlist, 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 shouldn't 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.
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 = '2003-07-03';
Increment the sales count of the salesperson who manages the account for Acme Corporation, using the FROM clause syntax:
UPDATE employees SET sales_count = sales_count + 1 FROM accounts WHERE accounts.name = 'Acme Corporation' AND employees.id = accounts.sales_person;
Perform the same operation, using a sub-select in the WHERE clause:
UPDATE employees SET sales_count = sales_count + 1 WHERE id = (SELECT sales_person 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;
This command conforms to the SQL standard, except that the FROM clause is a PostgreSQL extension.
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 PostgreSQL interprets FROM. Be careful when porting applications that use this extension.