some oracle sql

1. select (case when s_mark <> 80 then '优异'

when s_mark <= 80 and s_mark >= 60 then '优秀'

when s_mark <= 59 and s_mark >= 40 then '良好'

when s_mark <>= 20 then '一般'

when s_mark <= 19 and s_mark > 0 then '较差' from student_scores

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2. case和decode (P61)语句有什么区别？分别在什么场合使用？

SELECT CASE SIGN(5 - 5) WHEN 1 THEN 'Is Positive' WHEN -1 THEN 'Is Negative' ELSE 'Is Zero' END FROM DUAL;

select decode (条件，结果1，输出1，结果2，输出2)

SELECT DECODE(SIGN(5-9), 1, 'Is Positive', -1, 'Is Negative', 'Is Zero')FROM　DUAL

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