sql: Categorize SQL Statements

[jseldess]

Our statements list isn't huge yet, but it still might be helpful to organize the list based on whether a statement concerns data definition or data manipulation. Might also want to add DCL (data control, e.g., grant), TCL (transaction control), and Session Control statements.

https://www.cockroachlabs.com/docs/sql-statements.html

Reference:

• https://en.wikipedia.org/wiki/Data_definition_language
• https://en.wikipedia.org/wiki/Data_manipulation_language
• https://en.wikipedia.org/wiki/Data_control_language
• http://www.orafaq.com/faq/what_are_the_difference_between_ddl_dml_and_dcl_commands
• https://docs.oracle.com/cd/B14117_01/server.101/b10759/statements_1001.htm#i2099112

[jseldess]

@knz covered this in #1008