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   _____     ___      _____     _____    _____    
  / ___//   / _ \\   / ____||  |  ___|| |  __ \\  
  \___ \\  | / \ || / //---`'  | ||__   | |  \ || 
  /    //  | \_/ || \ \\___    | ||__   | |__/ || 
 /____//    \___//   \_____||  |_____|| |_____//  
`-----`     `---`     `----`   `-----`   -----`

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發送短信 : Functional dependencies with context dependent Null values in relational databases