Oracle instr

select instr('abc','a') from dual;
select instr('abc','bc') from dual;
select instr('abc ab','a',1,2) from dual;
select instr('abcab','a',-1,2) from dual;
select instr('abc','d') from dual;

分别返回

1

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5

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0

select instr('abcab','a',-1,2) from dual; 第三个参数-1，表示从字符串右面开始查找