1、nvl(expr1,expr2)
如exp1是空值，则返回exp2,否则返回expr1;
2、nvl2(expr1,expr2,expr3)
如果exp1是空值，则返回expr3,否则返回expr2;
3、coalesce(expr[,expr1]...)
返回参数里面第一个非空值;
with test as
( select 'c11' col_1, '' col_2, 'c31' col_3 from dual union all
select '' col_1, 'c21' col_2, 'c32' col_3 from dual union all
select 'c13' col_1, 'c22' col_2, '' col_3 from dual union all
select '' col_1, 'c23' col_2, 'c33' col_3 from dual union all
select 'c14' col_1, '' col_2, 'c34' col_3 from dual union all
select 'c15' col_1, '' col_2, '' col_3 from dual
)
select col_1, nvl(col_1, col_1) exp_1,
col_2, nvl2(col_2,col_2||',','is null') exp_2,
col_3, coalesce(col_1, col_2, col_3) exp_3
from test;
COL_1 EXP_1 COL_2 EXP_2 COL_3 EXP_3
--------- ----- ------- ------- ----- -----
c11 c11 is null c31 c11
c21 c21, c32 c21
c13 c13 c22 c22, c13
c23 c23, c33 c23
c14 c14 is null c34 c14
c15 c15 is null c15
6 rows selected