doc/src/sgml/dml.sgml

   1 <!-- $PostgreSQL$ -->
   2
   3 <chapter id="dml">
   4  <title>Data Manipulation</title>
   5
   6  <remark>
   7   This chapter is still quite incomplete.
   8  </remark>
   9
  10  <para>
  11   The previous chapter discussed how to create tables and other
  12   structures to hold your data.  Now it is time to fill the tables
  13   with data.  This chapter covers how to insert, update, and delete
  14   table data.  We also introduce ways to effect automatic data changes
  15   when certain events occur: triggers and rewrite rules.  The chapter
  16   after this will finally explain how to extract your long-lost data
  17   from the database.
  18  </para>
  19
  20  <sect1 id="dml-insert">
  21   <title>Inserting Data</title>
  22
  23   <indexterm zone="dml-insert">
  24    <primary>inserting</primary>
  25   </indexterm>
  26
  27   <indexterm zone="dml-insert">
  28    <primary>INSERT</primary>
  29   </indexterm>
  30
  31   <para>
  32    When a table is created, it contains no data.  The first thing to
  33    do before a database can be of much use is to insert data.  Data is
  34    conceptually inserted one row at a time.  Of course you can also
  35    insert more than one row, but there is no way to insert less than
  36    one row.  Even if you know only some column values, a
  37    complete row must be created.
  38   </para>
  39
  40   <para>
  41    To create a new row, use the <xref linkend="sql-insert"
  42    endterm="sql-insert-title"> command.  The command requires the
  43    table name and column values.  For
  44    example, consider the products table from <xref linkend="ddl">:
  45 <programlisting>
  46 CREATE TABLE products (
  47     product_no integer,
  48     name text,
  49     price numeric
  50 );
  51 </programlisting>
  52    An example command to insert a row would be:
  53 <programlisting>
  54 INSERT INTO products VALUES (1, 'Cheese', 9.99);
  55 </programlisting>
  56    The data values are listed in the order in which the columns appear
  57    in the table, separated by commas.  Usually, the data values will
  58    be literals (constants), but scalar expressions are also allowed.
  59   </para>
  60
  61   <para>
  62    The above syntax has the drawback that you need to know the order
  63    of the columns in the table.  To avoid this you can also list the
  64    columns explicitly.  For example, both of the following commands
  65    have the same effect as the one above:
  66 <programlisting>
  67 INSERT INTO products (product_no, name, price) VALUES (1, 'Cheese', 9.99);
  68 INSERT INTO products (name, price, product_no) VALUES ('Cheese', 9.99, 1);
  69 </programlisting>
  70    Many users consider it good practice to always list the column
  71    names.
  72   </para>
  73
  74   <para>
  75    If you don't have values for all the columns, you can omit some of
  76    them.  In that case, the columns will be filled with their default
  77    values.  For example:
  78 <programlisting>
  79 INSERT INTO products (product_no, name) VALUES (1, 'Cheese');
  80 INSERT INTO products VALUES (1, 'Cheese');
  81 </programlisting>
  82    The second form is a <productname>PostgreSQL</productname>
  83    extension.  It fills the columns from the left with as many values
  84    as are given, and the rest will be defaulted.
  85   </para>
  86
  87   <para>
  88    For clarity, you can also request default values explicitly, for
  89    individual columns or for the entire row:
  90 <programlisting>
  91 INSERT INTO products (product_no, name, price) VALUES (1, 'Cheese', DEFAULT);
  92 INSERT INTO products DEFAULT VALUES;
  93 </programlisting>
  94   </para>
  95
  96   <para>
  97    You can insert multiple rows in a single command:
  98 <programlisting>
  99 INSERT INTO products (product_no, name, price) VALUES
 100     (1, 'Cheese', 9.99),
 101     (2, 'Bread', 1.99),
 102     (3, 'Milk', 2.99);
 103 </programlisting>
 104   </para>
 105
 106   <tip>
 107    <para>
 108     When inserting a lot of data at the same time, considering using
 109     the <xref linkend="sql-copy" endterm="sql-copy-title"> command.
 110     It is not as flexible as the <xref linkend="sql-insert"
 111     endterm="sql-insert-title"> command, but is more efficient. Refer
 112     to <xref linkend="populate"> for more information on improving
 113     bulk loading performance.
 114    </para>
 115   </tip>
 116  </sect1>
 117
 118  <sect1 id="dml-update">
 119   <title>Updating Data</title>
 120
 121   <indexterm zone="dml-update">
 122    <primary>updating</primary>
 123   </indexterm>
 124
 125   <indexterm zone="dml-update">
 126    <primary>UPDATE</primary>
 127   </indexterm>
 128
 129   <para>
 130    The modification of data that is already in the database is
 131    referred to as updating.  You can update individual rows, all the
 132    rows in a table, or a subset of all rows.  Each column can be
 133    updated separately; the other columns are not affected.
 134   </para>
 135
 136   <para>
 137    To update existing rows, use the <xref linkend="sql-update"
 138    endterm="sql-update-title"> command.  This requires
 139    three pieces of information:
 140    <orderedlist spacing="compact">
 141     <listitem>
 142      <para>The name of the table and column to update</para>
 143     </listitem>
 144
 145     <listitem>
 146      <para>The new value of the column</para>
 147     </listitem>
 148
 149     <listitem>
 150      <para>Which row(s) to update</para>
 151     </listitem>
 152    </orderedlist>
 153   </para>
 154
 155   <para>
 156    Recall from <xref linkend="ddl"> that SQL does not, in general,
 157    provide a unique identifier for rows.  Therefore it is not
 158    always possible to directly specify which row to update.
 159    Instead, you specify which conditions a row must meet in order to
 160    be updated.  Only if you have a primary key in the table (independent of
 161    whether you declared it or not) can you reliably address individual rows
 162    by choosing a condition that matches the primary key.
 163    Graphical database access tools rely on this fact to allow you to
 164    update rows individually.
 165   </para>
 166
 167   <para>
 168    For example, this command updates all products that have a price of
 169    5 to have a price of 10:
 170 <programlisting>
 171 UPDATE products SET price = 10 WHERE price = 5;
 172 </programlisting>
 173     This might cause zero, one, or many rows to be updated.  It is not
 174     an error to attempt an update that does not match any rows.
 175   </para>
 176
 177   <para>
 178    Let's look at that command in detail. First is the key word
 179    <literal>UPDATE</literal> followed by the table name.  As usual,
 180    the table name can be schema-qualified, otherwise it is looked up
 181    in the path.  Next is the key word <literal>SET</literal> followed
 182    by the column name, an equal sign, and the new column value.  The
 183    new column value can be any scalar expression, not just a constant.
 184    For example, if you want to raise the price of all products by 10%
 185    you could use:
 186 <programlisting>
 187 UPDATE products SET price = price * 1.10;
 188 </programlisting>
 189    As you see, the expression for the new value can refer to the existing
 190    value(s) in the row.  We also left out the <literal>WHERE</literal> clause.
 191    If it is omitted, it means that all rows in the table are updated.
 192    If it is present, only those rows that match the
 193    <literal>WHERE</literal> condition are updated.  Note that the equals
 194    sign in the <literal>SET</literal> clause is an assignment while
 195    the one in the <literal>WHERE</literal> clause is a comparison, but
 196    this does not create any ambiguity.  Of course, the
 197    <literal>WHERE</literal> condition does
 198    not have to be an equality test.  Many other operators are
 199    available (see <xref linkend="functions">).  But the expression
 200    needs to evaluate to a Boolean result.
 201   </para>
 202
 203   <para>
 204    You can update more than one column in an
 205    <command>UPDATE</command> command by listing more than one
 206    assignment in the <literal>SET</literal> clause.  For example:
 207 <programlisting>
 208 UPDATE mytable SET a = 5, b = 3, c = 1 WHERE a &gt; 0;
 209 </programlisting>
 210   </para>
 211  </sect1>
 212
 213  <sect1 id="dml-delete">
 214   <title>Deleting Data</title>
 215
 216   <indexterm zone="dml-delete">
 217    <primary>deleting</primary>
 218   </indexterm>
 219
 220   <indexterm zone="dml-delete">
 221    <primary>DELETE</primary>
 222   </indexterm>
 223
 224   <para>
 225    So far we have explained how to add data to tables and how to
 226    change data.  What remains is to discuss how to remove data that is
 227    no longer needed.  Just as adding data is only possible in whole
 228    rows, you can only remove entire rows from a table.  In the
 229    previous section we explained that SQL does not provide a way to
 230    directly address individual rows.  Therefore, removing rows can
 231    only be done by specifying conditions that the rows to be removed
 232    have to match.  If you have a primary key in the table then you can
 233    specify the exact row.  But you can also remove groups of rows
 234    matching a condition, or you can remove all rows in the table at
 235    once.
 236   </para>
 237
 238   <para>
 239    You use the <xref linkend="sql-delete" endterm="sql-delete-title">
 240    command to remove rows; the syntax is very similar to the
 241    <command>UPDATE</command> command.  For instance, to remove all
 242    rows from the products table that have a price of 10, use:
 243 <programlisting>
 244 DELETE FROM products WHERE price = 10;
 245 </programlisting>
 246   </para>
 247
 248   <para>
 249    If you simply write:
 250 <programlisting>
 251 DELETE FROM products;
 252 </programlisting>
 253    then all rows in the table will be deleted!  Caveat programmer.
 254   </para>
 255  </sect1>
 256 </chapter>