PostgreSQL:

COMMENT

Topic

COMMENT ON
{
AGGREGATE agg_name (agg_type [, ...] ) |
CAST (source_type AS target_type) |
COLLATION object_name |
COLUMN relation_name.column_name |
CONSTRAINT constraint_name ON table_name |
CONVERSION object_name |
DATABASE object_name |
DOMAIN object_name |
EXTENSION object_name |
FOREIGN DATA WRAPPER object_name |
FOREIGN TABLE object_name |
FUNCTION function_name ( [ [ argmode ] [ argname ] argtype [, ...] ] ) |
INDEX object_name |
LARGE OBJECT large_object_oid |
OPERATOR operator_name (left_type, right_type) |
OPERATOR CLASS object_name USING index_method |
OPERATOR FAMILY object_name USING index_method |
[ PROCEDURAL ] LANGUAGE object_name |
ROLE object_name |
RULE rule_name ON table_name |
SCHEMA object_name |
SEQUENCE object_name |
SERVER object_name |
TABLE object_name |
TABLESPACE object_name |
TEXT SEARCH CONFIGURATION object_name |
TEXT SEARCH DICTIONARY object_name |
TEXT SEARCH PARSER object_name |
TEXT SEARCH TEMPLATE object_name |
TRIGGER trigger_name ON table_name |
TYPE object_name |
VIEW object_name
} IS 'text

Example

COMMENT ON TABLE mytable IS 'This is my table.';
COMMENT ON TABLE mytable IS NULL;
COMMENT ON AGGREGATE my_aggregate (double precision) IS 'Computes sample
variance';
COMMENT ON CAST (text AS int4) IS 'Allow casts from text to int4';
COMMENT ON COLLATION "fr_CA" IS 'Canadian French';
COMMENT ON COLUMN my_table.my_column IS 'Employee ID number';
COMMENT ON CONVERSION my_conv IS 'Conversion to UTF8';
COMMENT ON CONSTRAINT bar_col_cons ON bar IS 'Constrains column col';
COMMENT ON DATABASE my_database IS 'Development Database';