2本のダッシュ:

-- These are comments

2本のスラッシュ:

// These are comments

 -- This expression concatenates first and last names for customers: 
FIRST_NAME -- First names from the CUST table 
|| // Concat symbol 
LAST_NAME // Last names from the CUST table 
// Joe Smith Aug 18 1998 

FIRST_NAME || LAST_NAME 

-- This expression concatenates first and last names for customers: 
FIRST_NAME -- First names from the CUST table 
|| // Concat symbol 
LAST_NAME // Last names from the CUST table 
Joe Smith Aug 18 1998