src/sgml/dml.sgml

<!-- doc/src/sgml/dml.sgml -->

<chapter id="dml">
 <title>Data Manipulation</title>

 <para>
  The previous chapter discussed how to create tables and other
  structures to hold your data.  Now it is time to fill the tables
  with data.  This chapter covers how to insert, update, and delete
  table data.  The chapter
  after this will finally explain how to extract your long-lost data
  from the database.
 </para>

 <sect1 id="dml-insert">
  <title>Inserting Data</title>

  <indexterm zone="dml-insert">
   <primary>inserting</primary>
  </indexterm>

  <indexterm zone="dml-insert">
   <primary>INSERT</primary>
  </indexterm>

  <para>
   When a table is created, it contains no data.  The first thing to
   do before a database can be of much use is to insert data.  Data is
   conceptually inserted one row at a time.  Of course you can also
   insert more than one row, but there is no way to insert less than
   one row.  Even if you know only some column values, a
   complete row must be created.
  </para>

  <para>
   To create a new row, use the <xref linkend="sql-insert"/>
   command.  The command requires the
   table name and column values.  For
   example, consider the products table from <xref linkend="ddl"/>:
<programlisting>
CREATE TABLE products (
    product_no integer,
    name text,
    price numeric
);
</programlisting>
   An example command to insert a row would be:
<programlisting>
INSERT INTO products VALUES (1, 'Cheese', 9.99);
</programlisting>
   The data values are listed in the order in which the columns appear
   in the table, separated by commas.  Usually, the data values will
   be literals (constants), but scalar expressions are also allowed.
  </para>

  <para>
   The above syntax has the drawback that you need to know the order
   of the columns in the table.  To avoid this you can also list the
   columns explicitly.  For example, both of the following commands
   have the same effect as the one above:
<programlisting>
INSERT INTO products (product_no, name, price) VALUES (1, 'Cheese', 9.99);
INSERT INTO products (name, price, product_no) VALUES ('Cheese', 9.99, 1);
</programlisting>
   Many users consider it good practice to always list the column
   names.
  </para>

  <para>
   If you don't have values for all the columns, you can omit some of
   them.  In that case, the columns will be filled with their default
   values.  For example:
<programlisting>
INSERT INTO products (product_no, name) VALUES (1, 'Cheese');
INSERT INTO products VALUES (1, 'Cheese');
</programlisting>
   The second form is a <productname>PostgreSQL</productname>
   extension.  It fills the columns from the left with as many values
   as are given, and the rest will be defaulted.
  </para>

  <para>
   For clarity, you can also request default values explicitly, for
   individual columns or for the entire row:
<programlisting>
INSERT INTO products (product_no, name, price) VALUES (1, 'Cheese', DEFAULT);
INSERT INTO products DEFAULT VALUES;
</programlisting>
  </para>

  <para>
   You can insert multiple rows in a single command:
<programlisting>
INSERT INTO products (product_no, name, price) VALUES
    (1, 'Cheese', 9.99),
    (2, 'Bread', 1.99),
    (3, 'Milk', 2.99);
</programlisting>
  </para>

  <para>
   It is also possible to insert the result of a query (which might be no
   rows, one row, or many rows):
<programlisting>
INSERT INTO products (product_no, name, price)
  SELECT product_no, name, price FROM new_products
    WHERE release_date = 'today';
</programlisting>
   This provides the full power of the SQL query mechanism (<xref
   linkend="queries"/>) for computing the rows to be inserted.
  </para>

  <tip>
   <para>
    When inserting a lot of data at the same time, consider using
    the <xref linkend="sql-copy"/> command.
    It is not as flexible as the <xref linkend="sql-insert"/>
    command, but is more efficient. Refer
    to <xref linkend="populate"/> for more information on improving
    bulk loading performance.
   </para>
  </tip>
 </sect1>

 <sect1 id="dml-update">
  <title>Updating Data</title>

  <indexterm zone="dml-update">
   <primary>updating</primary>
  </indexterm>

  <indexterm zone="dml-update">
   <primary>UPDATE</primary>
  </indexterm>

  <para>
   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.
  </para>

  <para>
   To update existing rows, use the <xref linkend="sql-update"/>
   command.  This requires
   three pieces of information:
   <orderedlist spacing="compact">
    <listitem>
     <para>The name of the table and column to update</para>
    </listitem>

    <listitem>
     <para>The new value of the column</para>
    </listitem>

    <listitem>
     <para>Which row(s) to update</para>
    </listitem>
   </orderedlist>
  </para>

  <para>
   Recall from <xref linkend="ddl"/> 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.
  </para>

  <para>
   For example, this command updates all products that have a price of
   5 to have a price of 10:
<programlisting>
UPDATE products SET price = 10 WHERE price = 5;
</programlisting>
    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.
  </para>

  <para>
   Let's look at that command in detail. First is the key word
   <literal>UPDATE</literal> 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 <literal>SET</literal> followed
   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:
<programlisting>
UPDATE products SET price = price * 1.10;
</programlisting>
   As you see, the expression for the new value can refer to the existing
   value(s) in the row.  We also left out the <literal>WHERE</literal> clause.
   If it is omitted, it means that all rows in the table are updated.
   If it is present, only those rows that match the
   <literal>WHERE</literal> condition are updated.  Note that the equals
   sign in the <literal>SET</literal> clause is an assignment while
   the one in the <literal>WHERE</literal> clause is a comparison, but
   this does not create any ambiguity.  Of course, the
   <literal>WHERE</literal> condition does
   not have to be an equality test.  Many other operators are
   available (see <xref linkend="functions"/>).  But the expression
   needs to evaluate to a Boolean result.
  </para>

  <para>
   You can update more than one column in an
   <command>UPDATE</command> command by listing more than one
   assignment in the <literal>SET</literal> clause.  For example:
<programlisting>
UPDATE mytable SET a = 5, b = 3, c = 1 WHERE a &gt; 0;
</programlisting>
  </para>
 </sect1>

 <sect1 id="dml-delete">
  <title>Deleting Data</title>

  <indexterm zone="dml-delete">
   <primary>deleting</primary>
  </indexterm>

  <indexterm zone="dml-delete">
   <primary>DELETE</primary>
  </indexterm>

  <para>
   So far we have explained how to add data to tables and how to
   change data.  What remains is to discuss how to remove data that is
   no longer needed.  Just as adding data is only possible in whole
   rows, you can only remove entire rows from a table.  In the
   previous section we explained that SQL does not provide a way to
   directly address individual rows.  Therefore, removing rows can
   only be done by specifying conditions that the rows to be removed
   have to match.  If you have a primary key in the table then you can
   specify the exact row.  But you can also remove groups of rows
   matching a condition, or you can remove all rows in the table at
   once.
  </para>

  <para>
   You use the <xref linkend="sql-delete"/>
   command to remove rows; the syntax is very similar to the
   <command>UPDATE</command> command.  For instance, to remove all
   rows from the products table that have a price of 10, use:
<programlisting>
DELETE FROM products WHERE price = 10;
</programlisting>
  </para>

  <para>
   If you simply write:
<programlisting>
DELETE FROM products;
</programlisting>
   then all rows in the table will be deleted!  Caveat programmer.
  </para>
 </sect1>

 <sect1 id="dml-returning">
  <title>Returning Data from Modified Rows</title>

  <indexterm zone="dml-returning">
   <primary>RETURNING</primary>
  </indexterm>

  <indexterm zone="dml-returning">
   <primary>INSERT</primary>
   <secondary>RETURNING</secondary>
  </indexterm>

  <indexterm zone="dml-returning">
   <primary>UPDATE</primary>
   <secondary>RETURNING</secondary>
  </indexterm>

  <indexterm zone="dml-returning">
   <primary>DELETE</primary>
   <secondary>RETURNING</secondary>
  </indexterm>

  <para>
   Sometimes it is useful to obtain data from modified rows while they are
   being manipulated.  The <command>INSERT</command>, <command>UPDATE</command>,
   and <command>DELETE</command> commands all have an
   optional <literal>RETURNING</literal> clause that supports this.  Use
   of <literal>RETURNING</literal> avoids performing an extra database query to
   collect the data, and is especially valuable when it would otherwise be
   difficult to identify the modified rows reliably.
  </para>

  <para>
   The allowed contents of a <literal>RETURNING</literal> clause are the same as
   a <command>SELECT</command> command's output list
   (see <xref linkend="queries-select-lists"/>).  It can contain column
   names of the command's target table, or value expressions using those
   columns.  A common shorthand is <literal>RETURNING *</literal>, which selects
   all columns of the target table in order.
  </para>

  <para>
   In an <command>INSERT</command>, the data available to <literal>RETURNING</literal> is
   the row as it was inserted.  This is not so useful in trivial inserts,
   since it would just repeat the data provided by the client.  But it can
   be very handy when relying on computed default values.  For example,
   when using a <link linkend="datatype-serial"><type>serial</type></link>
   column to provide unique identifiers, <literal>RETURNING</literal> can return
   the ID assigned to a new row:
<programlisting>
CREATE TABLE users (firstname text, lastname text, id serial primary key);

INSERT INTO users (firstname, lastname) VALUES ('Joe', 'Cool') RETURNING id;
</programlisting>
   The <literal>RETURNING</literal> clause is also very useful
   with <literal>INSERT ... SELECT</literal>.
  </para>

  <para>
   In an <command>UPDATE</command>, the data available to <literal>RETURNING</literal> is
   the new content of the modified row.  For example:
<programlisting>
UPDATE products SET price = price * 1.10
  WHERE price &lt;= 99.99
  RETURNING name, price AS new_price;
</programlisting>
  </para>

  <para>
   In a <command>DELETE</command>, the data available to <literal>RETURNING</literal> is
   the content of the deleted row.  For example:
<programlisting>
DELETE FROM products
  WHERE obsoletion_date = 'today'
  RETURNING *;
</programlisting>
  </para>

  <para>
   If there are triggers (<xref linkend="triggers"/>) on the target table,
   the data available to <literal>RETURNING</literal> is the row as modified by
   the triggers.  Thus, inspecting columns computed by triggers is another
   common use-case for <literal>RETURNING</literal>.
  </para>

 </sect1>
</chapter>