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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