㈠ 函數VALUE和COALESCE到底有沒有區別
Purpose
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 NVLreturns expr1.
The arguments expr1 and expr2 can have any data type. If their data types are different, then Oracle Database implicitly converts one to the other. If they cannot be converted implicitly, then the database returns an error. The implicit conversion is implemented as follows:
• If expr1 is character data, then Oracle Database converts expr2 to the data type of expr1 before comparing them and returns VARCHAR2 in the character set of expr1.
• If expr1 is numeric, then Oracle Database determines which argument has the highest numeric precedence, implicitly converts the other argument to that data type, and returns that data type.
Examples
SQL> select * from scott.procts;
LIST_PRICE MIN_PRICE
---------- ----------
10000 8000
20000
30000 30000
SQL> select min_price,nvl(min_price,0) from scott.procts;
MIN_PRICE NVL(MIN_PRICE,0)
---------- ----------------
8000 8000
0
30000 30000
---------------------------------------------------------------------------------------------------------------------------------------
nvl2用法為nvl2(expr1,expr2,expr3)，其作用是判斷expr1是否為null，若不為null，返回expr2，為空返回expr3。
nvl(expr1,expr2)等同於nvl2(expr1,expr1,expr2)。
官方文檔用法解釋如下：
NVL2
Syntax
De.ion of nvl2.gif follows
Purpose
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 returnsexpr2. If expr1 is null, then NVL2 returns expr3.
The argument expr1 can have any data type. The arguments expr2 and expr3 can have any data types except LONG.
If the data types of expr2 and expr3 are different, then Oracle Database implicitly converts one to the other. If they cannot be converted implicitly, then the database returns an error. If expr2 is character or numeric data, then the implicit conversion is implemented as follows:
• If expr2 is character data, then Oracle Database converts expr3 to the data type of expr2 before returning a value unless expr3 is a null constant. In that case, a data type conversion is not necessary, and the database returns VARCHAR2 in the character set of expr2.
•
If expr2 is numeric data, then Oracle Database determines which argument has the highest numeric precedence, implicitly converts the other argument to that data type, and returns that data type.
Examples
SQL> select list_price,www.hbbz08.com min_price,nvl2(min_price,min_price,list_price) nv2_m from scott.procts;
LIST_PRICE MIN_PRICE NV2_M
---------- ---------- ----------
10000 8000 8000
20000 20000
30000 30000 30000
---------------------------------------------------------------------------------------------------------------------------------------
nullif用法為nullif(expr1,expr2)，其作用是判斷expr1與expr2是否相等，若相等則返回null，否則返回expr1。
官方文檔用法解釋如下：
NULLIF
Syntax
De.ion of nullif.gif follows
Purpose
NULLIF compares expr1 and expr2. If they are equal, then the function returns null. If they are not equal, then the function returns expr1. You cannot specify the literal NULL for expr1.
If both arguments are numeric data types, then Oracle Database determines the argument with the higher numeric precedence, implicitly converts the other argument to that data type, and returns that data type. If the arguments are not numeric, then they must be of the same data type, or Oracle returns an error.
The NULLIF function is logically equivalent to the following CASE expression:
CASE WHEN expr1 = expr2 THEN NULL ELSE expr1 END
Examples
SQL> select list_price,min_price,nullif(list_price,min_price) from scott.procts;
LIST_PRICE MIN_PRICE NULLIF(LIST_PRICE,MIN_PRICE)
---------- ---------- ----------------------------
10000 8000 10000
20000 20000
30000 30000
---------------------------------------------------------------------------------------------------------------------------------------
coalesce用法為coalesce(expr1,expr2……exprn)，其作用是在expr1,expr2……exprn這列表達式中查找第一個不為null的值且返回該值。如果都為null，則返回null。
官方文檔用法解釋如下：
COALESCE
Syntax
De.ion of coalesce.gif follows
Purpose
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.
Oracle Database uses short-circuit evaluation. The database evaluates each expr value and determines whether it is NULL, rather than evaluating all of theexpr values before determining whether any of them is NULL.
If all occurrences of expr are numeric data type or any nonnumeric data type that can be implicitly converted to a numeric data type, then Oracle Database determines the argument with the highest numeric precedence, implicitly converts the remaining arguments to that data type, and returns that data type.
This function is a generalization of the NVL function.
You can also use COALESCE as a variety of the CASE expression. For example,
COALESCE(expr1, expr2)
is equivalent to:
CASE WHEN expr1 IS NOT NULL THEN expr1 ELSE expr2 END
Similarly,
COALESCE(expr1, expr2, ..., exprn)
where n >= 3, is equivalent to:
CASE WHEN expr1 IS NOT NULL THEN expr1
ELSE COALESCE (expr2, ..., exprn) END
Examples
SQL> select list_price,min_price,coalesce(list_price,min_price) from scott.procts;
LIST_PRICE MIN_PRICE COALESCE(LIST_PRICE,MIN_PRICE)
---------- ---------- ------------------------------
10000 8000 10000
20000 20000
30000 30000 30000
到此，nvl、nvl2、nullif、coalesce四個函數用法我們都了解了，看似很簡單，但實際結合應用起來，就容易犯混，如下例子：
examples:
Examine the data in the LIST_PRICE and MIN_PRICE columns of the PRODUCTS table:
LIST_PRICE MIN_PRICE
10000 8000
20000
30000 30000
Which two expressions give the same output? (Choose two.)
A. NVL(NULLIF(list_price, min_price), 0)
B. NVL(COALESCE(list_price, min_price), 0)
C. NVL2(COALESCE(list_price, min_price), min_price, 0)
D. COALESCE(NVL2(list_price, list_price, min_price), 0)
Answer: BD
乍看你呢很快給出答案么，下面給出答案解析。
A. NVL(NULLIF(list_price, min_price), 0) 查詢結果如下：
SQL> select NVL(NULLIF(list_price, min_price), 0) from scott.procts;
NVL(NULLIF(LIST_PRICE,MIN_PRICE),0)
-----------------------------------
10000
20000
0
B. NVL(COALESCE(list_price, min_price), 0) 查詢結果如下：
SQL> select NVL(COALESCE(list_price, min_price), 0) from scott.procts;
NVL(COALESCE(LIST_PRICE,MIN_PRICE),0)
-------------------------------------
10000
20000
30000
C. NVL2(COALESCE(list_price, min_price), min_price, 0)查詢結果如下：
SQL> select NVL2(COALESCE(list_price, min_price), min_price, 0) from scott.procts;
NVL2(COALESCE(LIST_PRICE,MIN_PRICE),MIN_PRICE,0)
------------------------------------------------
8000
30000
D. COALESCE(NVL2(list_price, list_price, min_price), 0) 查詢結果如下：
SQL> select COALESCE(NVL2(list_price, list_price, min_price), 0) from scott.procts;
COALESCE(NVL2(LIST_PRICE,LIST_PRICE,MIN_PRICE),0)
-------------------------------------------------
10000
20000
30000
由此可見，選BD
㈡ COALESCE(MAX(meta_id),0)+1，請問這個SQL語句是什麼意思
表達式：COALESCE(MAX(meta_id),0)+1
其意思為選取欄位"meta_id"的最大值+1，如果該最大值為Null（空值），則將空值替換為0，然後+1
COALESCE函數簡要說明：
COALESCE (expression_1, expression_2, ...,expression_n)依次參考各參數表達式，遇到非null值即停止並返回該值。如果所有的表達式都是空值，最終將返回一個空值。
㈢ SQL Server ISNULL函數和Coalesce函數替換空值的區別
SELECT COALESCE('',0)結果0
SELECT COALESCE(' ',0) 結果0
SELECT COALESCE(null,0) 結果0
SELECT COALESCE(123,0) 結果123
SELECT ISNULL('',0) 結果''
SELECT ISNULL(null,0) 結果0
SELECT ISNULL(123,0)結果123
由結果結果可以看出COALESCE函數對於空值處理和NULL值都起作用。
㈣ SQL中 nvl（）、coalesce（）、decode（）這三個函數，如果只是判斷非空的話，哪一個效率相比較高
nvl只支持2個參數，這是oracle特有的
coalesce支持不定參數，coalesce(bonus,0,1)應該寫成coalesce(bonus,0)，最後1個不用寫，這是ansi標准函數，在多次外連接時尤其有用，譬如 t1 full join t2 on t1.id=t2.id full join t3 on coalesce(t1.id,t2.id)=t3.id full join t4 on coalesce(t1.id,t2.id,t3.id)=t4.id
decode判斷不了非空
㈤ 如何使用Oracle的COALESCE函數和nvl函數
NVL是Oracle PL/SQL中的一個函數。它的格式是NVL( string1, replace_with)。它的功能是如果string1為NULL，則NVL函數返回replace_with的值，否則返回string1的值，如果兩個參數都為NULL ，則返回NULL。
注意事項：string1和replace_with必須為同一數據類型，除非顯式的使用TO_CHAR函數進行類型轉換。
例：NVL(TO_CHAR(numeric_column), 'some string') 其中numeric_column代指某個數字類型的值。
例：nvl(yanlei777,0) > 0
NVL(yanlei777, 0) 的意思是 如果 yanlei777 是NULL， 則取 0值