update(5)
NAME
UPDATE - update rows of a table
SYNOPSIS
UPDATE [ ONLY ] table [ [ AS ] alias ]
SET { column = { expression | DEFAULT } |
( column [, ...] ) = ( { expression | DEFAULT } [, ...] ) } [, ...]
[ FROM fromlist ]
[ WHERE condition ]
[ RETURNING * | output_expression [ AS output_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 men-
tioned 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 addi-
tional 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.
PARAMETERS
table The name (optionally schema-qualified) of the table to update.
alias A substitute name for the target table. When an alias is pro-
vided, 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 [select(5)] 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.
OUTPUTS
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.
NOTES
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 tar-
get 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 tar-
get 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 tar-
get 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.
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 = '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 save-
points.
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 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 inde-
pendent expressions.
Some other database systems offer a FROM option in which the target ta-
ble is supposed to be listed again within FROM. That is not how Post-
greSQL interprets FROM. Be careful when porting applications that use
this extension.
SQL - Language Statements 2008-06-08 UPDATE()
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