6.2. 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 update existing rows, use the UPDATE command. This requires 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 Chapter 5 that SQL does not, in general, provide a unique identifier for rows. Therefore it is not always 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 (independent of 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 might 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
followed by the table name. As usual,
the table name can be schema-qualified, otherwise it is looked up
in the path. Next is the key word
by the column name, an equal 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
condition are updated. Note that the equals
sign in the
clause is an assignment while
the one in the
clause is a comparison, but
this does not create any ambiguity. Of course, the
not have to be an equality test. Many other operators are
). But the expression
needs to evaluate to a Boolean result.
You can update more than one column in an
command by listing more than one
assignment in the
clause. For example:
UPDATE mytable SET a = 5, b = 3, c = 1 WHERE a > 0;