A. 求SQL语言中COALESCE字符函数的用法
功能：返回其参数中的第一个非空表达式，当你要在n个字段中选取某一个非空值
可以用它，比如下面语句
select Coalesce(null,null,1,2,null)union
select Coalesce(null,11,12,13,null)union
select Coalesce(111,112,113,114,null)
返回结果：
1
11
111
B. SQL中 nvl（）、coalesce（）、decode（）这三个函数，如果只是判断非空的话，哪一个效率相比较高
nvl只支持2个参数，这是oracle特有的
coalesce支持不定参数，coalesce(bonus,0,1)应该写成coalesce(bonus,0)，最后1个不用写，这是ansi标准函数，在多次外连接时尤其有用，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判断不了非空
C. SQL中 nvl（）coalesce（）decode（）这三个函数是不是递进包含关系
nvl(bonus,0) 意思是 如果 bonus is null ， 那么返回 0， 否则返回 bonus
coalesce(bonus,0,1) 意思是 返回 参数列表中， 第一个非 空的数据。
也就是相当于， 如果 bonus is null ， 那么返回 0， 否则返回 bonus。
这里的最后一个参数 1， 目测是打酱油的。
coalesce(bonus,null,0) 意思是 返回 参数列表中， 第一个非 空的数据。
也就是相当于， 如果 bonus is null ， 那么第2个参数还是 null, 最后返回第3个参数 0
decode(name,‘apple’,0) 意思是， 如果 name = 'apple' 那么返回 0
否则的话 , 就是返回 null 了。
D. SQL Server ISNULL函数和Coalesce函数替换空值的区别
SELECT COALESCE('',0)结果0
SELECT COALESCE(' ',0) 结果0
SELECT COALESCE(null,0) 结果0
SELECT COALESCE(123,0) 结果123
SELECT ISNULL('',0) 结果''
SELECT ISNULL(null,0) 结果0
SELECT ISNULL(123,0)结果123
由结果结果可以看出COALESCE函数对于空值处理和NULL值都起作用。
E. COALESCE(MAX(meta_id),0)+1，请问这个SQL语句是什么意思
表达式：COALESCE(MAX(meta_id),0)+1
其意思为选取字段"meta_id"的最大值+1，如果该最大值为Null（空值），则将空值替换为0，然后+1
COALESCE函数简要说明：
COALESCE (expression_1, expression_2, ...,expression_n)依次参考各参数表达式，遇到非null值即停止并返回该值。如果所有的表达式都是空值，最终将返回一个空值。
F. 函数VALUE和COALESCE到底有没有区别
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用法为nvl2(expr1,expr2,expr3)，其作用是判断expr1是否为null，若不为null，返回expr2，为空返回expr3。
nvl(expr1,expr2)等同于nvl2(expr1,expr1,expr2)。
官方文档用法解释如下：
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用法为nullif(expr1,expr2)，其作用是判断expr1与expr2是否相等，若相等则返回null，否则返回expr1。
官方文档用法解释如下：
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用法为coalesce(expr1,expr2……exprn)，其作用是在expr1,expr2……exprn这列表达式中查找第一个不为null的值且返回该值。如果都为null，则返回null。
官方文档用法解释如下：
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四个函数用法我们都了解了，看似很简单，但实际结合应用起来，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
乍看你呢很快给出答案么，下面给出答案解析。
A. NVL(NULLIF(list_price, min_price), 0) 查询结果如下：
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) 查询结果如下：
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)查询结果如下：
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) 查询结果如下：
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
由此可见，选BD
G. oracle存储过程中带空值的输入怎么写COALESCE()函数用错了吗
空值 ，你直接用NVL不就行了.
open cusers for select * from C_USERS where logintime >= NVL(start_time,logintime)
and logintime <= NVL(end_time,logintime)
and username = NVL(name,username)
and truename = NVL(tname,truename);
H. COALESCE 是否是SQL标准函数
在oracle中不是，
server中，没测试。
数据库系统函数，是可以自己查看到的。
I. SQL语句：COALESCE(t1.CONTENT,'') <>'' 为什么不等于 t1.CONTENT<>NULL我觉得是一样的。。。
null 在数据库中 表示 不可知
你不能 用 = null 、 <> null 等等 进行判断。
判断的结果 同样是： 不可知（不确定）
所以所有的返回结果都是 false
可以 t1.CONTENT is not NULL 、 t1.CONTENT is NULL 进行判断
J. 求SQL语言中COALESCE字符函数的用法！！！！！
功能：返回其参数中的第一个非空表达式，当你要在n个字段中选取某一个非空值
可以用它，比如下面语句
select Coalesce(null,null,1,2,null)union
select Coalesce(null,11,12,13,null)union
select Coalesce(111,112,113,114,null)
返回结果：
1
11
111