关于count(case when col_name='a' then 'b' else)

你的sql没有写对。
正常的应该是：
select count(case when col='a' then 'b' end) counts from tab_name;
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log
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[TEST@ora1] SQL>select * from test4;

COL COL1
--- ----
a 3
c r
v 3

[TEST@ora1] SQL>select count(case when col='a' then 'b' else) counts from test4;

select count(case when col='a' then 'b' else) counts from test4
*
ERROR at line 1:
ORA-00936: missing expression

[TEST@ora1] SQL>select count(case when col='a' then 'b' end) counts from test4;

COUNTS
----------
1
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原因：
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[TEST@ora1] SQL>select nvl(case when col='a' then 'b' end,'NULL') from test4;

NVL(
---