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InterSystems SQL Reference
COALESCE
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A function that returns the value of the first expression that is not NULL.
Synopsis
COALESCE(expression,expression)
Arguments
expression A series of expressions to be evaluated. Multiple expressions are specified as a comma-separated list. This expression list has a limit of 140 expressions.
Description
The COALESCE function evaluates a list of expressions in left-to-right order and returns the value of the first non-NULL expression. If all expressions evaluate to NULL, NULL is returned.
A string is returned unchanged; leading and trailing blanks are retained. A number is returned in canonical form, with leading and trailing zeros removed.
For further details on NULL handling, refer to the NULL and the Empty String section of “Language Elements” in Using InterSystems SQL.
Data Type of Returned Value
Non-numeric expressions (such as strings or dates) must all be of the same data type, and return a value of that data type. Specifying expressions with incompatible data types results in an SQLCODE -378 error with a Datatype mismatch error message. You can use the CAST function to convert an expression to a compatible data type.
Numeric expressions may be of different data types. If you specify numeric expressions with different data types, the data type returned is the expression data type most compatible with all of the possible result values, the data type with the highest data type precedence.
A literal value (string, number, or NULL) is treated as data type VARCHAR. If you specify only two expressions, a literal value is compatible with a numeric expression: if the first expression is the numeric expression, its data type is returned; if the first expression is a literal value, the VARCHAR data type is returned.
NULL Handling Functions Compared
The following table shows the various SQL comparison functions. Each function returns one value if the logical comparison tests True (A same as B) and another value if the logical comparison tests False (A not same as B). These functions allow you to perform NULL logical comparisons. You cannot specify NULL in an actual equality (or non-equality) condition comparison.
SQL Function Comparison Test Return Value
COALESCE(ex1,ex2,...) ex = NULL for each argument
True tests next ex argument. If all ex arguments are True (NULL), returns NULL.
False returns ex
IFNULL(ex1,ex2) [two-argument form] ex1 = NULL
True returns ex2
False returns NULL
IFNULL(ex1,ex2) [three-argument form] ex1 = NULL
True returns ex2
False returns ex3
{fn IFNULL(ex1,ex2)} ex1 = NULL
True returns ex2
False returns ex1
ISNULL(ex1,ex2) ex1 = NULL
True returns ex2
False returns ex1
NVL(ex1,ex2) ex1 = NULL
True returns ex2
False returns ex1
NULLIF(ex1,ex2) ex1 = ex2
True returns NULL
False returns ex1
Examples
The following Embedded SQL example takes a series of host variable values and returns the first (value d) that is not NULL. Note that the ObjectScript empty string ("") is translated as NULL in InterSystems SQL:
  SET (a,b,c,e)=""
  SET d="firstdata"
  SET f="nextdata"
  &sql(SELECT COALESCE(:a,:b,:c,:d,:e,:f) INTO :x)
   IF SQLCODE'=0 {
     WRITE !,"Error code ",SQLCODE }
   ELSE {
     WRITE !,"The first non-null value is: ",x }
The following example compares the values of two columns in left-to-right order and returns the value of the first non-NULL column. The FavoriteColors column is NULL for some rows; the Home_State column is never NULL. For COALESCE to compare the two, FavoriteColors must be cast as a string:
SELECT TOP 25 Name,FavoriteColors,Home_State,
COALESCE(CAST(FavoriteColors AS VARCHAR),Home_State) AS CoalesceCol
FROM Sample.Person
The following Dynamic SQL example compares COALESCE to the other NULL-processing functions:
  ZNSPACE "SAMPLES"
  SET myquery = "SELECT TOP 50 %ID,"_
                "IFNULL(FavoriteColors,'blank') AS Ifn2Col,"_
                "IFNULL(FavoriteColors,'blank','value') AS Ifn3Col,"_
                "COALESCE(CAST(FavoriteColors AS VARCHAR),Home_State) AS CoalesceCol,"_
                "ISNULL(FavoriteColors,'blank') AS IsnullCol,"_
                "NULLIF(FavoriteColors,$LISTBUILD('Orange')) AS NullifCol,"_
                "NVL(FavoriteColors,'blank') AS NvlCol"_
                " FROM Sample.Person"
  SET tStatement = ##class(%SQL.Statement).%New()
  SET qStatus = tStatement.%Prepare(myquery)
    IF qStatus'=1 {WRITE "%Prepare failed:" DO $System.Status.DisplayError(qStatus) QUIT}
  SET rset = tStatement.%Execute()
  DO rset.%Display()
  WRITE !,"End of data"
See Also