• Conversion Functions
  例子：

-- cast
SELECT cast('10' as int);
+---------------+
|CAST(10 AS INT)|
+---------------+
|             10|
+---------------+

• Conditional Functions
  例子：

-- coalesce
SELECT coalesce(NULL, 1, NULL);
+-----------------------+
|coalesce(NULL, 1, NULL)|
+-----------------------+
|                      1|
+-----------------------+

-- if
SELECT if(1 < 2, 'a', 'b');
+-------------------+
|(IF((1 < 2), a, b))|
+-------------------+
|                  a|
+-------------------+

-- ifnull
SELECT ifnull(NULL, array('2'));
+----------------------+
|ifnull(NULL, array(2))|
+----------------------+
|                   [2]|
+----------------------+

-- nanvl
SELECT nanvl(cast('NaN' as double), 123);
+-------------------------------+
|nanvl(CAST(NaN AS DOUBLE), 123)|
+-------------------------------+
|                          123.0|
+-------------------------------+

-- nullif
SELECT nullif(2, 2);
+------------+
|nullif(2, 2)|
+------------+
|        NULL|
+------------+

-- nvl
SELECT nvl(NULL, array('2'));
+-------------------+
|nvl(NULL, array(2))|
+-------------------+
|                [2]|
+-------------------+

-- nvl2
SELECT nvl2(NULL, 2, 1);
+----------------+
|nvl2(NULL, 2, 1)|
+----------------+
|               1|
+----------------+

-- when
SELECT CASE WHEN 1 > 0 THEN 1 WHEN 2 > 0 THEN 2.0 ELSE 1.2 END;
+-----------------------------------------------------------+
|CASE WHEN (1 > 0) THEN 1 WHEN (2 > 0) THEN 2.0 ELSE 1.2 END|
+-----------------------------------------------------------+
|                                                        1.0|
+-----------------------------------------------------------+

SELECT CASE WHEN 1 < 0 THEN 1 WHEN 2 > 0 THEN 2.0 ELSE 1.2 END;
+-----------------------------------------------------------+
|CASE WHEN (1 < 0) THEN 1 WHEN (2 > 0) THEN 2.0 ELSE 1.2 END|
+-----------------------------------------------------------+
|                                                        2.0|
+-----------------------------------------------------------+

SELECT CASE WHEN 1 < 0 THEN 1 WHEN 2 < 0 THEN 2.0 END;
+--------------------------------------------------+
|CASE WHEN (1 < 0) THEN 1 WHEN (2 < 0) THEN 2.0 END|
+--------------------------------------------------+
|                                              NULL|
+--------------------------------------------------+