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NAME

       UPDATE - update rows of a table

SYNOPSIS

       UPDATE [ ONLY ] table SET column = { expression | DEFAULT } [, ...]
           [ FROM fromlist ]
           [ WHERE condition ]

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.

       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.

       column The name of a column in table.  The column name can be qualified
              with a subfield name or array subscript, if needed.

       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(7)] 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.

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).

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
       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 indeterminancy, 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’;

       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;

COMPATIBILITY

       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.