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| -rw-r--r-- | doc/src/sgml/ddl.sgml | 30 |
1 files changed, 17 insertions, 13 deletions
diff --git a/doc/src/sgml/ddl.sgml b/doc/src/sgml/ddl.sgml index 6d29859ed97..3ae7d241e7b 100644 --- a/doc/src/sgml/ddl.sgml +++ b/doc/src/sgml/ddl.sgml @@ -1,4 +1,4 @@ -<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.36 2005/01/08 01:44:05 tgl Exp $ --> +<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.37 2005/01/09 17:47:30 tgl Exp $ --> <chapter id="ddl"> <title>Data Definition</title> @@ -1279,23 +1279,22 @@ WHERE c.altitude > 500 and c.tableoid = p.oid; <programlisting> ALTER TABLE products ADD COLUMN description text; </programlisting> - The new column will initially be filled with null values in the - existing rows of the table. + The new column is initially filled with whatever default + value is given (null if you don't specify a <literal>DEFAULT</> clause). </para> <para> - You can also define a constraint on the column at the same time, + You can also define constraints on the column at the same time, using the usual syntax: <programlisting> ALTER TABLE products ADD COLUMN description text CHECK (description <> ''); </programlisting> - A new column cannot have a not-null constraint since the column - initially has to contain null values. But you can add a not-null - constraint later. Also, you cannot define a default value on a - new column. According to the SQL standard, this would have to - fill the new columns in the existing rows with the default value, - which is not implemented yet. But you can adjust the column - default later on. + In fact all the options that can be applied to a column description + in <command>CREATE TABLE</> can be used here. Keep in mind however + that the default value must satisfy the given constraints, or the + <literal>ADD</> will fail. Alternatively, you can add + constraints later (see below) after you've filled in the new column + correctly. </para> </sect2> @@ -1390,12 +1389,17 @@ ALTER TABLE products ALTER COLUMN product_no DROP NOT NULL; <programlisting> ALTER TABLE products ALTER COLUMN price SET DEFAULT 7.77; </programlisting> + Note that this doesn't affect any existing rows in the table, it + just changes the default for future <command>INSERT</> commands. + </para> + + <para> To remove any default value, use <programlisting> ALTER TABLE products ALTER COLUMN price DROP DEFAULT; </programlisting> - This is equivalent to setting the default to null, at least in - <productname>PostgreSQL</>. As a consequence, it is not an error + This is equivalent to setting the default to null. + As a consequence, it is not an error to drop a default where one hadn't been defined, because the default is implicitly the null value. </para> |