Escaping ' in Access SQL

问题:

I'm trying to do a domain lookup in vba with something like this:

DLookup("island", "villages", "village = '" & txtVillage & "'")

This works fine until txtVillage is something like Dillon's Bay, when the apostrophe is taken to be a single quote, and I get a run-time error.

I've written a trivial function that escapes single quotes - it replaces "'" with "''". This seems to be something that comes up fairly often, but I can't find any reference to a built-in function that does the same. Have I missed something?

回答1:

The "Replace" function should do the trick. Based on your code above:

DLookup("island", "villages", "village = '" & Replace(txtVillage, "'", "''") & "'")

回答2:

It's worse than you think. Think about what would happen if someone entered a value like this, and you haven't escaped anything:

'); DROP TABLE [YourTable]

Not pretty.

The reason there's no built in function to simply escape an apostrophe is because the correct way to handle this is to use query parameters. For an Ole/Access style query you'd set this as your query string:

DLookup("island", "village", "village = ? ")

And then set the parameter separately. I don't know how you go about setting the parameter value from vba, though.

回答3:

Though the shorthand domain functions such as DLookup are tempting, they have their disadvantages. The equivalent Jet SQL is something like

SELECT FIRST(island)
FROM villages
WHERE village = ?;

If you have more than one matching candidate it will pick the 'first' one, the definition of 'first' is implementation (SQL engine) dependent and undefined for the Jet/ACE engine IIRC. Do you know which one would be first? If you don’t then steer clear of DLookup :)

[For interest, the answer for Jet/ACE will either be the minimum value based on the clusterd index at the time the database file was last compacted or the first (valid time) inserted value if the database has never been compacted. Clustered index is in turn determined by the PRIAMRY KEY if persent otherwise a UNIQUE constraint or index defined on NOT NULL columns, otherwise the first (valid time) inserted row. What if there is more than one UNIQUE constraint or index defined on NOT NULL columns, which one would be used for clustering? I've no idea! I trust you get the idea that 'first' is not easy to determine, even when you know how!]

I've also seen advice from Microsoft to avoid using domain aggregate functions from an optimization point of view:

Information about query performance in an Access database http://support.microsoft.com/kb/209126

"Avoid using domain aggregate functions, such as the DLookup function... the Jet database engine cannot optimize queries that use domain aggregate functions"

If you choose to re-write using a query you can then take advantage of the PARAMETERS syntax, or you may prefer the Jet 4.0/ACE PROCEDURE syntax e.g. something like

CREATE PROCEDURE GetUniqueIslandName
(
   :village_name VARCHAR(60)
)
AS 
SELECT V1.island_name
  FROM Villages AS V1
 WHERE V1.village_name = :village_name
       AND EXISTS 
       (
        SELECT V2.village_name
          FROM Villages AS V2
         WHERE V2.village_name = V1.village_name
         GROUP 
            BY V2.village_name
        HAVING COUNT(*) = 1
       );

This way you can use the engine's own functionality -- or at least that of its data providers -- to escape all characters (not merely double- and single quotes) as necessary.

回答4:

I believe access can use Chr$(34) and happily have single quotes/apostrophes inside.
eg

DLookup("island", "villages", "village = " & chr$(34) & nonEscapedString & chr$(34))

Though then you'd have to escape the chr$(34) (")

You can use the Replace function.

Dim escapedString as String

escapedString = Replace(nonescapedString, "'", "''")

回答5:

Parametrized queries such as Joel Coehoorn suggested are the way to go, instead of doing concatenation in query string. First - avoids certain security risks, second - I am reasonably certain it takes escaping into engine's own hands and you don't have to worry about that.

回答6:

But then, it should be like this (with one more doublequote each):

sSQL = "SELECT * FROM tblTranslation WHERE fldEnglish=""" & myString & """;"

Or what I prefer:

Make a function to escape single quotes, because "escaping" with "[]" would not allow these characters in your string...

Public Function fncSQLStr(varStr As Variant) As String

If IsNull(varStr) Then
        fncSQLStr = ""
    Else
        fncSQLStr = Replace(Trim(varStr), "'", "''")
    End If

End Function

I use this function for all my SQL-queries, like SELECT, INSERT and UPDATE (and in the WHERE clause as well...)

strSQL = "INSERT INTO tbl" & 
    " (fld1, fld2)" & _
    " VALUES ('" & fncSQLStr(str1) & "', '" & fncSQLStr(Me.tfFld2.Value) & "');"

or

strSQL = "UPDATE tbl" & _
    " SET fld1='" & fncSQLStr(str1) & "', fld2='" & fncSQLStr(Me.tfFld2.Value) & "'" & _
    " WHERE fld3='" & fncSQLStr(str3) & "';"

回答7:

By the way, here's my EscapeQuotes function

Public Function EscapeQuotes(s As String) As String

    If s = "" Then
        EscapeQuotes = ""
    ElseIf Left(s, 1) = "'" Then
        EscapeQuotes = "''" & EscapeQuotes(Mid(s, 2))
    Else
        EscapeQuotes = Left(s, 1) & EscapeQuotes(Mid(s, 2))
    End If

End Function

回答8:

For who having trouble with single quotation and Replace function, this line may save your day ^o^

Replace(result, "'", "''", , , vbBinaryCompare)

回答9:

put brackets around the criteria that might have an apostrophe in it.

SOmething like:

DLookup("island", "villages", "village = '[" & txtVillage & "]'")

They might need to be outside the single quotes or just around txtVillage like:

DLookup("island", "villages", "village = '" & [txtVillage] & "'")

But if you find the right combination, it will take care of the apostrophe.

Keith B

回答10:

My solution is much simpler. Originally, I used this SQL expression to create an ADO recordset:

Dim sSQL as String
sSQL="SELECT * FROM tblTranslation WHERE fldEnglish='" & myString & "';"

When myString had an apostrophe in it, like Int'l Electrics, my program would halt. Using double quotes solved the problem.

sSQL="SELECT * FROM tblTranslation WHERE fldEnglish="" & myString & "";"