1 COALESCE

語法：COALESCE(expr1, expr2, ..., exprn) n>=2

作用：COALESCE returns the first non-null expr in the expression list. You must specify at least
two expressions. If all occurrences of expr evaluate to null, then the function returns
null.

COALESCE用於返回第一個非空的表達式.表達式只是兩個.如果所有的表達式為null，則返回null.

COALESCE(expr1, expr2)等價于 CASE WHEN expr1 IS NOT NULL THEN expr1 ELSE expr2 END

例子：

SELECT product_id, list_price, min_price,
            COALESCE(0.9*list_price, min_price, 5) "Sale"
  FROM product_information
  WHERE supplier_id = 102050
  ORDER BY product_id;

2 NVL

語法：NVL（expr1,expr2）

NVL lets you replace null (returned as a blank) with a string in the results of a query. If
expr1 is null, then NVL returns expr2. If expr1 is not null, then NVL returns expr1.

NVL 如果expr1為空，則返回expr2，不為空，就返回expr1.

例子：

SELECT last_name, NVL(TO_CHAR(commission_pct), 'Not Applicable') commission
    FROM employees
    WHERE last_name LIKE 'B%'
    ORDER BY last_name;

3 NVL2

語法：NVL2（expr1,expr2,expr3）

作用：NVL2 lets you determine the value returned by a query based on whether a specified
expression is null or not null. If expr1 is not null, then NVL2 returns expr2. If expr1 is
null, then NVL2 returns expr3.

如果expr1的值為空，則返回expr2,不為空則返回expr3.

SELECT last_name, salary,
            NVL2(commission_pct, salary + (salary * commission_pct), salary) income
  FROM employees
  WHERE last_name like 'B%'
  ORDER BY last_name;

4 DECODE

語法：DECODE(expr,search1,result1,search2,result2...searchN,resultN,default)

作用：DECODE compares expr to each search value one by one. If expr is equal to a search,
then Oracle Database returns the corresponding result. If no match is found, then
Oracle returns default. If default is omitted, then Oracle returns null.

比較expr和search的值，如果相等，則返回result，否則返回default.

就是說如果expr等於search1，則返回result1的值，等於search2，則返回result2...如果都不滿足就返回default值.

例子：

SELECT product_id,
DECODE (warehouse_id, 1, 'Southlake',
                                    2, 'San Francisco',
                                    3, 'New Jersey',
                                    4, 'Seattle',
                                       'Non domestic') "Location"
FROM inventories
WHERE product_id < 1775
ORDER BY product_id, "Location";