SQL conditional functions (#4775)

content/influxdb/cloud-iox/reference/sql/functions/conditional.md

@@ -0,0 +1,97 @@
+---
+title: SQL conditional functions
+list_title: Conditional functions
+description: >
+  Use conditional functions to conditionally handle null values in SQL queries.
+menu:
+  influxdb_cloud_iox:
+    name: Conditional
+    parent: sql-functions    
+weight: 306
+---
+
+The InfluxDB SQL implementation supports the following conditional functions for
+conditionally handling _null_ values:
+
+- [coalesce](#coalesce)
+- [nullif](#nullif)
+
+## coalesce
+
+Returns the first of its arguments that is not _null_.
+Returns _null_ if all arguments are _null_.
+This function is often used to substitute a default value for _null_ values.
+
+```sql
+coalesce(expression1[, ..., expression_n])
+```
+
+##### Arguments
+
+- **expression1, expression_n**:
+  Column or literal value to use if previous expressions are _null_.
+  Pass as many expression arguments as necessary. 
+
+{{< expand-wrapper >}}
+{{% expand "View `coalesce` query example" %}}
+
+```sql
+SELECT
+  val1,
+  val2,
+  val3,
+  coalesce(val1, val2, val3, 'quz') AS coalesce
+FROM
+  (values ('foo', 'bar', 'baz'),
+          (NULL, 'bar', 'baz'),
+          (NULL, NULL, 'baz'),
+          (NULL, NULL, NULL)
+  ) data(val1, val2, val3)
+```
+
+| val1 | val2 | val3 | coalesce |
+| :--: | :--: | :--: | :------: |
+| foo  | bar  | baz  |   foo    |
+|      | bar  | baz  |   bar    |
+|      |      | baz  |   baz    |
+|      |      |      |   quz    |
+
+{{% /expand %}}
+{{< /expand-wrapper >}}
+
+## nullif
+
+Returns _null_ if _expression1_ equals _expression2_; otherwise it returns _expression1_.
+This can be used to perform the inverse operation of [`coalesce`](#coalesce).
+
+```sql
+nullif(expression1, expression2)
+```
+
+##### Arguments
+
+- **expression1**: Column or literal value to compare and return if equal to expression2.
+- **expression2**: Column or literal value to compare to expression1.
+
+{{< expand-wrapper >}}
+{{% expand "View `nullif` query example" %}}
+
+```sql
+SELECT
+  value,
+  nullif(value, 'baz') AS nullif
+FROM
+  (values ('foo'),
+          ('bar'),
+          ('baz')
+  ) data(value)
+```
+
+| value | nullif |
+| :---- | :----- |
+| foo   | foo    |
+| bar   | bar    |
+| baz   |        |
+
+{{% /expand %}}
+{{< /expand-wrapper >}}