COMMENT (Comment-Retrieving Form) - Teradata Database - Teradata Vantage NewSQL Engine

Purpose

Retrieves a previously placed comment on a database object or definition.

For information about the comment-placing form of this SQL statement, see “COMMENT (Comment-Placing Form)” in Teradata Vantage™ SQL Data Definition Language Detailed Topics , B035-1184 .

Required Privileges

None.

Privileges Granted Automatically

None.

Syntax

Syntax Elements

object_kind_1

Mandatory database object kind specification.

The valid specifications for object_kind_1 are not all database objects. Triggers and views, for example, are definitions of actions or conceptual groups rather than database objects.

You must specify the following database object kinds to retrieve a comment for the kind of object they represent.

• COLUMN
• FUNCTION
• GLOP SET
• GROUP
• MACRO
• METHOD
• PROCEDURE
• PROFILE
• ROLE
• TRIGGER
• TYPE
• VIEW

object_kind_2

Optional database object kind specification.

You can specify the following database object kinds to retrieve a comment for the kind of object they represent, but they are optional.

• DATABASE
• TABLE
• USER
• FILE

The semantics of the COMMENT statement require that if you specify an object kind, you must specify the keyword TABLE for any hash or join index whose comment you want to retrieve; however, because TABLE is not a mandatory object kind specification, you are not required to specify it.

If you omit the object_kind when you retrieve a comment for a file, the object_name must contain the whole path to the file, for example, dbname.uifname.

database_name

user_name

Containing database or user for object_name if it is not contained by the current database or user.

You cannot specify a database name or user name if object_kind is GROUP.

For the rules to use in naming database objects, see Teradata Vantage™ SQL Fundamentals, B035-1141.

object_name

Name of the object for which a comment is to be retrieved, including:

• Parameter in a macro, stored procedure, or user-defined function.
• Column in a user base table, error table, hash index, join index, or view.
• Specific function, macro, profile, role, stored procedure, base table, error table, hash index, join index, trigger, or view name contained by a database or user.
• Database or user.
• UDT. You can retrieve a comment on a particular attribute of a structured UDT by specifying database_name.udt_name.attribute_name.
• A method. You must use the specific method name.
• A GLOP set.

If you do not precede the object name with an object kind keyword, the system attempts to deduce the object from the level of qualification in the name. Use the fully-qualified name to avoid ambiguity.

Let x.y.z indicate the qualification hierarchy, with x being the highest, or coarsest grained, level and z the lowest, or most finely grained.

If you specify a hierarchy level of x, the specified object is implied to be one of the following:

• Database
• User

If you specify a hierarchy level of x.y, the specified object is implied to be one of the following:

• Base table
• Error table
• GLOP set
• Hash index
• Join index
• Macro
• Profile
• Role
• Stored procedure
• Trigger
• User-defined function
• View within database or user x.

If you specify a hierarchy level of x.y.z, the specified object is implied to be one of the following:

• Macro parameter
• Stored procedure parameter
• Structured UDT attribute
• Table column
• UDF parameter
• View column within GLOP set, UDF, UDT, macro, stored procedure, profile, role, base table, trigger, or view y contained in database or user x.

ANSI Compliance

COMMENT is a Teradata extension to the ANSI SQL:2011 standard.

Rules for Using COMMENT (Comment-Retrieving Form)

Example: Retrieving a Comment

Suppose you have placed the following comment describing the name column in the employee table:

     COMMENT ON COLUMN employee.name
       IS 'Employee name, last name followed by first initial';

The following request retrieves this comment:

     COMMENT ON COLUMN employee.name;

The request returns the comment string as created by the previous request:

     Employee name, last name followed by first initial

Note that because the object kind for this request is COLUMN, you must specify the keyword COLUMN when you code it.

Example: Placing a Comment

Suppose you have defined a hash index named ord_hidx defined on the orders table, which is contained in the accounting database.

Because the object kind for this request is TABLE, representing a hash index, you are not required to specify TABLE in the request.

You could type the following comment-placing COMMENT request to define a comment on the hash index ord_hidx:

     COMMENT accounting.ord_hidx AS 'hash index on Orders';

Then if you were to perform the following comment-retrieving COMMENT request, the text that follows would be returned to you:

     COMMENT accounting.ord_hidx;
     
     hash index on orders