Which four statements correctly describe functions that are available in SQL? （）
- A INSTR returns the numeric position of a named character.
- B NVL2 returns the first non-null expression in the expression list.
- C TRUNCATE rounds the column, expression, or value to n decimal places.
- D DECODE translates an expression after comparing it to each search value.
- E TRIM trims the heading of trailing characters (or both) from a character string.
- F NVL compares two expressions and returns null if they are equal, or the first expression of they are not equal.
- G NULLIF compares twp expressions and returns null if they are equal, or the first expression if they are not equal.