oracle½ØÈ¡×Ö·û´®µÄº¯Êýsubstr

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substr( string, start_position, [ length ] )
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    substr('This is a test', 6, 2)          would return 'is'
    substr('This is a test', 6)              would return 'is a test'
    substr('TechOnTheNet', 1, 4)     would return 'Tech'
    substr('TechOnTheNet', -3, 3)    would return 'Net'
    substr('TechOnTheNet', -6, 3)    would return 'The'
    substr('TechOnTheNet', -8, 2)    would return 'On'
select substr('TechOnTheNet', -3, 2) from dual
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