|PostgreSQL Reference Manual - Volume 1 - SQL Language Reference|
by The PostgreSQL Global Development Group
Paperback (6"x9"), 716 pages
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4.2 Updating Data
The modification of data that is already in the database is referred to as updating. You can update individual rows, all the rows in a table, or a subset of all rows. Each column can be updated separately; the other columns are not affected.
To perform an update, you need three pieces of information:
- The name of the table and column to update,
- The new value of the column,
- Which row(s) to update.
Recall from section 3 Data Definition that SQL does not, in general, provide a unique identifier for rows. Therefore it is not necessarily possible to directly specify which row to update. Instead, you specify which conditions a row must meet in order to be updated. Only if you have a primary key in the table (no matter whether you declared it or not) can you reliably address individual rows, by choosing a condition that matches the primary key. Graphical database access tools rely on this fact to allow you to update rows individually.
For example, this command updates all products that have a price of 5 to have a price of 10:
UPDATE products SET price = 10 WHERE price = 5;
This may cause zero, one, or many rows to be updated. It is not an error to attempt an update that does not match any rows.
Let's look at that command in detail. First is the key word
UPDATE followed by the table name. As usual,
the table name may be schema-qualified, otherwise it is looked up
in the path. Next is the key word
by the column name, an equals sign and the new column value. The
new column value can be any scalar expression, not just a constant.
For example, if you want to raise the price of all products by 10%
you could use:
UPDATE products SET price = price * 1.10;
As you see, the expression for the new value can refer to the existing
value(s) in the row. We also left out the
If it is omitted, it means that all rows in the table are updated.
If it is present, only those rows that match the
WHERE condition are updated. Note that the equals
sign in the
SET clause is an assignment while
the one in the
WHERE clause is a comparison, but
this does not create any ambiguity. Of course, the
WHERE condition does
not have to be an equality test. Many other operators are
available (see section 7 Functions and Operators). But the expression
needs to evaluate to a Boolean result.
You can update more than one column in an
UPDATE command by listing more than one
assignment in the
SET clause. For example:
UPDATE mytable SET a = 5, b = 3, c = 1 WHERE a > 0;
|ISBN 0954612027||PostgreSQL Reference Manual - Volume 1 - SQL Language Reference||See the print edition|