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Find a query code for sql querying empty fields

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Post time: 2020-1-6 18:00:01
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Access was used more than a year ago.At that time, several fields were established in the database for future upgrades.
These four fields of ABCD were not used at the time, so the code of these fields was not written when writing the data. Now, these fields need to be enabled because of the upgrade. An error will occur because the record is empty. So at the beginning of the program, add the following code,

sql_.Format ("select * from customer management where IsNull [A]");
_bstr_t sql = sql_;
theApp.m_pRs = theApp.m_pCon-> Execute (sql, NULL, adCmdText);
while (! theApp.m_pRs-> adoEOF)
{
theApp.m_pRs-> Fields-> GetItem ("a")-> PutValue ("");
theApp.m_pRs-> Fields-> GetItem ("b")-> PutValue ("");
theApp.m_pRs-> Fields-> GetItem ("c")-> PutValue ("");
theApp.m_pRs-> Fields-> GetItem ("d")-> PutValue ("");
theApp.m_pRs-> Fields-> GetItem ("e")-> PutValue (renshishijian1);
// theApp.m_pRs-> Update ();
theApp.m_pRs-> MoveNext ();
}
But the SQL statement was not written correctly, and no relevant code was found on the Internet.
sql_.Format ("select * from customer management where A IsNull"); It is not correct to write this, please enlighten.
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Post time: 2020-1-14 20:09:01
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select * from customer management where A Is Null
or
select * from customer management where A = ""
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Post time: 2020-1-17 23:45:01
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select * from customer management where A Is Null
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Post time: 2020-1-17 23:54:01
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select * from customer management where IsNull (A)
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Post time: 2020-2-16 14:15:01
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First determine if it is empty OR NULL?
NULL
select * from customer management where IsNull (A)
air
select * from customer management where LEN (TRIM (A)) = 0
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Post time: 2020-6-1 13:30:01
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select * from customer management where [a] is null or [a] = '' "
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