The Item Class - MySQL

To us, the word Item means more than just “thingamabob”; it is a technical term with a precise definition in the context of our source code. Item is a class. Each instance of the Item class has:

• an analogue in the SQL language
• a value
• a data type descriptor

All of the following SQL “thingamabobs” are modeled in the Item class:

• literals
• column references
• session or global variables
• procedure variables
• parameters
• SQL functions (not a surprise since SQL functions have data types and return values).

In the function category we include operators such as + and ||, because operators are merely functions that return values. We also include operators such as = and LIKE, which are operators that return boolean values. Consider the following statement:

SELECT UPPER(column1) FROM t WHERE column2 = @x;

For this statement, MySQL will need to store a list of items for the select list ('column1' column reference and UPPER function), and a list of items for the WHERE clause ('column2' column reference and'@x' variable and '=' operator).

Terminology: an Item instance in a MySQL program roughly corresponds to a "site", which according to the standard_SQL definition is "a place that holds an instance of a value of a specified data type", Another word that you'll see often in MySQL code is "field", which means column reference, and the Item_field subclass is generally for column values that occur for the intersection of a row and column in a table.

MySQL's Item class is defined in .../sql/item.h, and its subclasses are defined in .../sql/item*.h (that is, in item.h, item_cmpfunc.h, item_func.h, item_geofunc.h, item_row.h, item_strfunc.h, item_subselect.h, item_sum.h, item_timefunc.h). Page-width limitations prevent us from displaying the whole tree, but these are the main Item subclasses, and the subclasses of the subclasses:

There's no formal classification of subclasses, but the main distinctions are by use (field, parameter,function) and by data type (num, string).

So, how does MySQL use items? You'll find that nearly every .cc program in the /sql directory makes some use of the Item class and its subclasses, so this list of programs is only partial and very general:

Whenever there's a need for an SQL operation that assigns, compares, aggregates, accepts, sends, or validates a site, you'll find a MySQL use of Item and its subclasses.