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????2.??=???????
??????=?????????????NULL?????????κ???????????=????????????NULL?????Ч????????????????<=>??
????'a' <=> NULL ??0   NULL<=> NULL ?ó? 1????=???????????=????????????? 'a'=NULL ?????NULL ????NULL = NULL ??????NULL?????????mysql????????е???????????????????????????????NULL???????????????塣

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????... WHERE col_a <=> ? ...
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????'a' IS NULL     ==> 'a' <=> NULL
????'a' IS NOT NULL ==> NOT('a' <=> NULL)
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????WHERE p.name IS NULL