
This function retrieves a column value from a row string given the name of
the table and column if such a column existed in the row.  If not, NULL is
returned.

row is a row string obtained by selecting _ROW from
some table, or a row identity string returned by
row_identity().  In this case
the column being retrieved must be part of the primary key of the table
the row identity string references.

Table_name is the name of the table that defines the column being retrieved.

Col_name is the name of the column.

The output parameter exists is optional.  If a
variable is specified here,
it is set to 1 if the table contains a column of the name requested.

If table_name and col_name specify a column that is not in the row,
NULL is returned. This can happen if the row comes from a table other than
the one specified in table_name.

is identical to

The direct column reference is faster, naturally.
