原表如下：
with t1 as(
select 1 as id, "a,b" as course
union
select 2 as id, "a,b,c" as course
union
select 3 as id, null as course
union
select 4 as id, "c" as course
)
select * from t1;
| id | course |
|---|---|
| 1 | a,b |
| 2 | a,b,c |
| 3 | NULL |
| 4 | c |
需要将 course 按照 ‘,‘ 拆分成下表：
| id | course |
|---|---|
| 1 | a |
| 1 | b |
| 2 | a |
| 2 | b |
| 2 | c |
| 3 | NULL |
| 4 | c |
方法如下：
with t1 as(
select 1 as