UPDATE — update rows of a table
UPDATE [ ONLY ]table
[ [ AS ]alias
] SET {column
= {expression
| DEFAULT } | (column
[, ...] ) = ( {expression
| DEFAULT } [, ...] ) } [, ...] [ FROMfromlist
] [ WHEREcondition
] [ RETURNING * |output_expression
[ ASoutput_name
] [, ...] ]
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.
The optional RETURNING
clause causes UPDATE
to compute and return value(s) based on each row actually updated.
Any expression using the table's columns, and/or columns of other
tables mentioned in FROM
, can be computed.
The new (post-update) values of the table's columns are used.
The syntax of the RETURNING
list is identical to that of the
output list of SELECT
.
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
.
table
The name (optionally schema-qualified) of the table to update.
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 — for example,
UPDATE tab SET tab.col = 1
is invalid.
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.
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.
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).
If the UPDATE
command contains a RETURNING
clause, the result will be similar to that of a SELECT
statement containing the columns and values defined in the
RETURNING
list, computed over the row(s) updated by the
command.
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';
Perform the same operation and return the updated entries:
UPDATE weather SET temp_lo = temp_lo+1, temp_hi = temp_lo+15, prcp = DEFAULT WHERE city = 'San Francisco' AND date = '2003-07-03' RETURNING temp_lo, temp_hi, prcp;
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 = '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
and RETURNING
clauses
are PostgreSQL extensions.
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 PostgreSQL interprets
FROM
. Be careful when porting applications that use this
extension.