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## What changes were proposed in this pull request?
This PR proposes `rand`/`randn` accept `null` as input in Scala/SQL and `LongType` as input in SQL. In this case, it treats the values as `0`.
So, this PR includes both changes below:
- `null` support
It seems MySQL also accepts this.
``` sql
mysql> select rand(0);
+---------------------+
| rand(0) |
+---------------------+
| 0.15522042769493574 |
+---------------------+
1 row in set (0.00 sec)
mysql> select rand(NULL);
+---------------------+
| rand(NULL) |
+---------------------+
| 0.15522042769493574 |
+---------------------+
1 row in set (0.00 sec)
```
and also Hive does according to [HIVE-14694](https://issues.apache.org/jira/browse/HIVE-14694)
So the codes below:
``` scala
spark.range(1).selectExpr("rand(null)").show()
```
prints..
**Before**
```
Input argument to rand must be an integer literal.;; line 1 pos 0
org.apache.spark.sql.AnalysisException: Input argument to rand must be an integer literal.;; line 1 pos 0
at org.apache.spark.sql.catalyst.analysis.FunctionRegistry$$anonfun$5.apply(FunctionRegistry.scala:465)
at org.apache.spark.sql.catalyst.analysis.FunctionRegistry$$anonfun$5.apply(FunctionRegistry.scala:444)
```
**After**
```
+-----------------------+
|rand(CAST(NULL AS INT))|
+-----------------------+
| 0.13385709732307427|
+-----------------------+
```
- `LongType` support in SQL.
In addition, it make the function allows to take `LongType` consistently within Scala/SQL.
In more details, the codes below:
``` scala
spark.range(1).select(rand(1), rand(1L)).show()
spark.range(1).selectExpr("rand(1)", "rand(1L)").show()
```
prints..
**Before**
```
+------------------+------------------+
| rand(1)| rand(1)|
+------------------+------------------+
|0.2630967864682161|0.2630967864682161|
+------------------+------------------+
Input argument to rand must be an integer literal.;; line 1 pos 0
org.apache.spark.sql.AnalysisException: Input argument to rand must be an integer literal.;; line 1 pos 0
at org.apache.spark.sql.catalyst.analysis.FunctionRegistry$$anonfun$5.apply(FunctionRegistry.scala:465)
at
```
**After**
```
+------------------+------------------+
| rand(1)| rand(1)|
+------------------+------------------+
|0.2630967864682161|0.2630967864682161|
+------------------+------------------+
+------------------+------------------+
| rand(1)| rand(1)|
+------------------+------------------+
|0.2630967864682161|0.2630967864682161|
+------------------+------------------+
```
## How was this patch tested?
Unit tests in `DataFrameSuite.scala` and `RandomSuite.scala`.
Author: hyukjinkwon <gurwls223@gmail.com>
Closes #15432 from HyukjinKwon/SPARK-17854.
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