#3153
implement subquery support for sub query in case when statement like
```
SELECT CASE
WHEN (
SELECT COUNT(*) / 2
FROM t
) > k4 THEN (
SELECT AVG(k4)
FROM t
)
ELSE (
SELECT SUM(k4)
FROM t
)
END AS kk4
FROM t;
```
this statement will be rewrite to
```
SELECT CASE
WHEN t1.a > k4 THEN t2.a
ELSE t3.a
END AS kk4
FROM t, (
SELECT COUNT(*) / 2 AS a
FROM t
) t1, (
SELECT AVG(k4) AS a
FROM t
) t2, (
SELECT SUM(k4) AS a
FROM t
) t3;
```
71bc815b20