SQL NULL values & COALESCE

SQL and columns with NULL values?

  • In today's database products, NULLs are used to represent both missing and inapplicable values.
  • Unknown pieces of information are called distinguished nulls—their values are not precisely known.
  • In relational database management systems, however, distinguished nulls are problematic in terms of both definition and implementation.
  • You cannot determine whether a NULL exactly matches any other value, even another NULL.
  • When you perform an arithmetic operation on an unknown value, the result can only be unknown.
  • However, Aggregate functions will omit the NULL values in the calculations.
  • To overcome the problem of NULL, you can set default values for columns when you CREATE or ALTER a table.
  • A good default for a character type column might be "unknown," while a good default for the date column might be today's date.
  • Another way to handle the NULL values is to a use functions that work with NULL values, which the function COALESCE() do.
    COALESCE Example:
    MySQL, Oracle, SQL server, PostgreSQL:
    select 	TELEPHONE_TYPE, CUSTOMER_ID,
            COALESCE(TELEPHONE_NUMBER,'Only Fax') as "Contact phone no."
       from telephones where TELEPHONE_TYPE='Unknown' ;
    The result should be:
    telephone_type customer_id Contact phone no.
    Unknown 10002 (678) 223-94
    Unknown 10004 (881) 141-54
    Unknown 10004 Only Fax
    Unknown 10005 (881) 141-54
    Unknown 10005 Only Fax
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