Use of comments

Use of table comments

Commenting on each table field helps users better understand the table and each field, facilitating coherent development among developers. Table comments are a favorable supplement to database design documents and a natural feature description.

You can add comments to a table by using two methods:

• Add comments to a table when you create the table,

  obclient> CREATE TABLE student(
  id INT PRIMARY KEY AUTO_INCREMENT COMMENT 'Student No.',
  nameVARCHAR(200) COMMENT 'Name',
  ageint COMMENT 'Age') COMMENT='Student information';
  

• After you create the table, use the ALTER statement to add comments,

  obclient> CREATE TABLE student( id INT PRIMARY KEY AUTO_INCREMENT , name VARCHAR(200) , age int);
  obclient> ALTER TABLE student COMMENT 'Student information';
  obclient> ALTER TABLE student MODIFY COLUMN id COMMENT 'Student No.';
  obclient> ALTER TABLE student MODIFY COLUMN name COMMENT 'Name';
  obclient> ALTER TABLE student MODIFY COLUMN age COMMENT 'Age';
  

  We recommend that you use the second method to add comments, that is, use the ALTER statement after the table is created. This method facilitates follow-up table maintenance.

Use of comments in SQL statements

OceanBase Database allows you to add comments in general SQL statements by using one of the following three methods:

• Start the comment with a number sign (#) and end it with a line break. The comment text cannot extend to a new line.

• Start the comment with two hyphens (--) and end it with a line break. The comment text cannot extend to a new line.

• Start the comment with a slash and an asterisk (/*) and end it with an asterisk and a slash (*/). The comment text can span multiple lines.

In particular, OceanBase Database can execute statements in /*! */ as SQL statements. If a syntax error occurs in /*! */, OceanBase Database processes it as a comment by default.

Comment format:

/*![Consecutive numeric characters representing the version]<Space><Any SQL statement>*/

Example:

• Create eight hash partitions for table t1.

  obclient> CREATE TABLE t1 (c1 INT PRIMARY KEY, c2 INT) /*!50100
      PARTITION BY HASH(c1) PARTITIONS 8*/;
  Query OK, 0 rows affected
  

• You can use ENABLE KEYS to update non-unique indexes. During batch insertion, you can use DISABLE KEYS to postpone the update until all indexes are inserted.

  obclient> /*!ALTER TABLE t1 DISABLE KEYS */;
  Query OK, 0 rows affected
  
  
  obclient> /*!ALTER TABLE t1 ENABLE KEYS */;
  Query OK, 0 rows affected
  

• The DROP IF EXISTS syntax of PL is supported.

  obclient> /*!50003 DROP PROCEDURE IF EXISTS */;
  Query OK, 0 rows affected