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:

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

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:

Item_ident (Item_field, Item_ref) 
Item_null
Item_num (Item_int, Item_real)
Item_param
Item_string (Item_static_string_func, Item_datetime, Item_empty_string)
Item_hex_string (Item_bin_string)
Item_result_field (all "item_func.h" "item_subselect.h" "item_sub.h" classes)
Item_copy_string
Item_cache (Item_cache_int, Item_cache_real, Item_cache_str, Item_cache_row)
Item_type_holder
Item_row

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:

sql_parse.cc:      Makes new items in add_field_to_list() 
item_sum.cc:       Uses item_func subclasses for COUNT, AVG, SUM 
item_buff.cc:      Where buffers for item values can be stored 
item_cmpfunc.cc:   Comparison functions with item_func subclasses 
item_create.cch:   For creating items that the lex might use 
item_subselect.cc: Subqueries are another type of function 
mysqld.cc:         When main() ends, it uses clean_up() for items 
opt_range.cc:      Uses field, compare-condition, and value subclasses 
procedure.cc:      Notice Procedure * has a pointer to an item list 
protocol.cc:       Uses send_fields() to pass item values back to users 
sys_var.cc:        System variables have Item associations too 
sql_base.cc:       Thread-specific Item searchers like find_field_in_table() 
sql_class.cc:      Look at cleanup_after_query() 
sql_delete.cc:     This (like sql_insert.cc etc.) has field references 
sql_error.cc:      Has one of many examples of SHOW's use of items 
sql_lex.cc:        Notice "add...to_list" functions 
sql_select.cc:     The largest program that uses items, apparently 
udf_example.cc:    The comments in this program are extensive

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.