as a developer and writing querys you offtn encounter in a situation where you want to replace a NULL in your result every RDBMS has its own system function to handle this today we will go through these functions in detail
We will start with SQL Server
In sql Server we have to functions one is ISNULL and other COALESCE
ISNULL is a bult in TSQL function develped by Microsoft , it replaces null value with scpecified value
ISNULL(Check expression (Column/variable/value), replacement expression(Column/Variable/value))
The replacement expression should be impilictly convertable to check expression data type, SQl server implicitly converts
replacement value to the data type of Check expression and returns the value if check expression is null
If we look at new column’s defnition it is a char datatype
However if you switch our replacement expression with check expression it throws an error saying
Msg 245, Level 16, State 1, Line 7
Conversion failed when converting the varchar value ‘a’ to data type int.
Which is not possible according to sqlserver
Sql server data type conversion chart
It can only have two input expressions
Where as if we look at COALESCE it takes multiple expressions and returns first not null value
The one big dfference between ISNULL and COALESCE is the return data type,
When using ISNULL if check expression is evaluated to NULL and return expression is NOT NULL and is different data type that of check expression SQL Server implictly converts return expression to the data type of check expressions
Where as COALESCE well it returns the first not null value but the data type it returns depends on the SQL Server data type precedence
SQL server data type precedence
NVL equavalent to ISNULL in SQLSERVER
NVL in ORACLE
NVL in Oracle works excatly as ISNULL in sql server
NVL (exp1 , exp2)
Exp1 is check expression and Exp2 is a replacement value
if Exp1 is NULL it returns Exp2 else it returns Exp1
Exp1 and Exp2 might not be same datatype just like in sql server but it should be implicitly convertble to the datatype of Exp1
But this implicit conversion is done in a smarter way when compared to isnull
expr1is character data, then Oracle Database converts
expr2to the datatype of
expr1before comparing them and returns
VARCHAR2in the character set of
expr1is numeric, then Oracle determines which argument has the highest numeric precedence, implicitly converts the other argument to that datatype, and returns that datatype.
Numeric precedence determines, for operations that support numeric datatypes, the datatype Oracle uses if the arguments to the operation have different datatypes.
BINARY_DOUBLE has the highest numeric precedence, followed by
BINARY_FLOAT, and finally by
NUMBER. Therefore, in any operation on multiple numeric values:
- If any of the operands is
BINARY_DOUBLE, then Oracle attempts to convert all the operands implicitly to
BINARY_DOUBLEbefore performing the operation.
- If none of the operands is
BINARY_DOUBLEbut any of the operands is
BINARY_FLOAT, then Oracle attempts to convert all the operands implicitly to
BINARY_FLOATbefore performing the operation.
- Otherwise, Oracle attempts to convert all the operands to
NUMBERbefore performing the operation.
Implicit Type Conversion Matrix
Table 2-10 Implicit Type Conversion Matrix
Oracle Implicit Conversion Matrix
IFNULLI() in MY SQL
IFNULL() is equavalent to NVL() in oracle and ISNULL() in Microsoft Sql server
12 thoughts on “ISNULL , COALESCE in SQLSERVER, NVL in ORACLE, IFNULL() in MYSQL”
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