Perl的MySQL查询访问超出了数组的边界

Question

我有以下过程中，我使用同步两个MySQL表，其中：

sub sync{ 
    my %tables = (
     'sitematrix_test' => 'sitematrix_sites' 
    ); 

    while(($key, $value) = each(%tables)){ 
     print "Matching columns $key : $value\n"; 

     my @data; 
     my $query = $db->prepare(" 
      SELECT $key.* FROM $key LEFT JOIN $value ON 
      $key.site_id = $value.site_id WHERE $value.site_id IS NULL; 
     ") 
      or die "Couldn't prepare statement: " . $db->errstr; 

     $query->execute() 
      or die "Couldn't execute statement: " . $query->errstr; 

     if($query->rows == 0){ 
      print "No updates have been made for $key : $value"; 
     } 
     else{ 
      #read the matching records and print them out 
      while(@data = $query->fetchrow_array()){ 
       print "@data\n"; 
      } 
     } 

     $query->finish; 
    } 

    $db->disconnect;  
} 

它给了以下错误：

Use of uninitialized value $data[3] in join or string at C:/Users/souzamor/workspace/Parser/synchronizer.pl line 69. 
Use of uninitialized value $data[4] in join or string at C:/Users/souzamor/workspace/Parser/synchronizer.pl line 69. 
Use of uninitialized value $data[5] in join or string at C:/Users/souzamor/workspace/Parser/synchronizer.pl line 69. 
Use of uninitialized value $data[6] in join or string at C:/Users/souzamor/workspace/Parser/synchronizer.pl line 69. 

可能有人请解释为什么它超出了数组的界限？

Answer 1

您在您的数据库中有NULL s;这些在Perl中转换为undef。警告来自以下行：

print "@data\n"; 

您在此处对数组进行了字符串化。例如：

perl -Mwarnings -e '@foo=(1, 2, undef, 3); print "@foo\n"' 
Use of uninitialized value $foo[2] in join or string at -e line 1. 
1 2 3 

如果你真的想字符串化整个阵列每一次打印，简单的解决将是undef秒值进行转换为空字符串：

while(my @data = map { defined($_) ? $_ : '' } $query->fetchrow_array()) { 

或者，如果你不” t需要打印全部每一行的数据，只需打印一个主键或者你知道的东西不会是NULL。