Clearify CHECK handling of unknown test values.

Karl O. Pinc

diff --git a/doc/src/sgml/ref/create_table.sgml b/doc/src/sgml/ref/create_table.sgml
index 771919f1646a6c1131553290b55f5b412c94f82a..511aacc73f3347b341e387165a0a7154005b8bbe 100644 (file)
--- a/doc/src/sgml/ref/create_table.sgml
+++ b/doc/src/sgml/ref/create_table.sgml
@@ -1,5 +1,5 @@
 
 
@@ -383,10 +383,13 @@ and table_constraint is:
      
       The CHECK clause specifies an expression producing a
       Boolean result which new or updated rows must satisfy for an
-      insert or update operation to succeed.  A check constraint
-      specified as a column constraint should reference that column's
-      value only, while an expression appearing in a table constraint
-      may reference multiple columns.
+      insert or update operation to succeed.  Expressions evaluating
+      to TRUE or UNKNOWN succeed.  Should any row of an insert or
+      update operation produce a FALSE result an error exception is
+      raised and the insert or update does not alter the database.  A
+      check constraint specified as a column constraint should
+      reference that column's value only, while an expression
+      appearing in a table constraint may reference multiple columns.