SQL: Implement NVL(expr1, expr2) (#35794)

Add NVL as alias to IFNULL as they have the same
behaviour. Add basic tests and docs.

Closes: #35782

docs/reference/sql/functions/conditional.asciidoc

@@ -118,3 +118,40 @@ include-tagged::{sql-specs}/docs.csv-spec[isNullReturnFirst]
 ----
 include-tagged::{sql-specs}/docs.csv-spec[isNullReturnSecond]
 ----
+
+
+[[sql-functions-conditional-nvl]]
+==== `NVL`
+
+.Synopsis
+[source, sql]
+----
+NVL ( expression<1>, expression<2> )
+----
+
+*Input*:
+
+<1> 1st expression
+
+<2> 2nd expression
+
+
+*Output*: 2nd expression if 1st expression is null, otherwise 1st expression.
+
+.Description
+
+Variant of <<sql-functions-conditional-coalesce>> with only two arguments.
+Returns the first of its arguments that is not null.
+If all arguments are null, then it returns `null`.
+
+
+
+["source","sql",subs="attributes,callouts,macros"]
+----
+include-tagged::{sql-specs}/docs.csv-spec[nvlReturnFirst]
+----
+
+["source","sql",subs="attributes,callouts,macros"]
+----
+include-tagged::{sql-specs}/docs.csv-spec[nvlReturnSecond]
+----

x-pack/plugin/sql/qa/src/main/resources/command.csv-spec

@@ -22,8 +22,9 @@ VAR_POP         |AGGREGATE
 COALESCE        |CONDITIONAL
 IFNULL          |CONDITIONAL
 ISNULL          |CONDITIONAL
+NVL             |CONDITIONAL
 DAY             |SCALAR
-DAYNAME         |SCALAR         
+DAYNAME         |SCALAR
 DAYOFMONTH      |SCALAR         
 DAYOFWEEK       |SCALAR         
 DAYOFYEAR       |SCALAR         

x-pack/plugin/sql/qa/src/main/resources/docs.csv-spec

@@ -199,8 +199,9 @@ VAR_POP         |AGGREGATE
 COALESCE        |CONDITIONAL
 IFNULL          |CONDITIONAL
 ISNULL          |CONDITIONAL
+NVL             |CONDITIONAL
 DAY             |SCALAR
-DAYNAME         |SCALAR         
+DAYNAME         |SCALAR
 DAYOFMONTH      |SCALAR         
 DAYOFWEEK       |SCALAR         
 DAYOFYEAR       |SCALAR         
@@ -1555,7 +1556,6 @@ search
 // end::ifNullReturnSecond
 ;
 
-
 isNullReturnFirst
 // tag::isNullReturnFirst
 SELECT ISNULL('elastic', null) AS "isnull";
@@ -1576,3 +1576,24 @@ SELECT ISNULL(null, 'search') AS "isnull";
 search
 // end::isNullReturnSecond
 ;
+
+nvlReturnFirst
+// tag::nvlReturnFirst
+SELECT NVL('elastic', null) AS "nvl";
+
+    nvl
+---------------
+elastic
+// end::nvlReturnFirst
+;
+
+
+nvlReturnSecond
+// tag::nvlReturnSecond
+SELECT NVL(null, 'search') AS "nvl";
+
+    nvl
+---------------
+search
+// end::nvlReturnSecond
+;

x-pack/plugin/sql/src/main/java/org/elasticsearch/xpack/sql/expression/function/FunctionRegistry.java

@@ -146,7 +146,7 @@ public class FunctionRegistry {
         // Scalar functions
         // conditional
         addToMap(def(Coalesce.class, Coalesce::new));
-        addToMap(def(IFNull.class, IFNull::new, "ISNULL"));
+        addToMap(def(IFNull.class, IFNull::new, "ISNULL", "NVL"));
         // Date
         addToMap(def(DayName.class, DayName::new, "DAYNAME"),
                 def(DayOfMonth.class, DayOfMonth::new, "DAYOFMONTH", "DAY", "DOM"),