Author: Lou Adler
This is a bit of a departure from my normal style of Q & A, but I thought it would
be useful for those of you who do a lot of database-related stuff.
Answer:
General Purpose Stuff
Face it, code reuse is big issue irrespective of programming in an object-oriented
environment. Especially, with database application programming, the stuff you do
over and over again can be a bit tedious. So, I've created a bunch of general
purpose functions that I use in my code - a lot.
An On-the-Fly SELECT Statement Generator
The first function CreateSelect creates a SELECT or SELECT DISTINCT statement that
can be used in whenever you create a TQuery for querying against a local table like
Paradox. It's most useful for when you want to create a query from a table at
runtime, but don't know the fields you want to include. For instance, I created an
adhoc querying tool where users could select the fields they wanted displayed in a
graph by dragging the field names from on list box into another. From there, I
passed the selected fields list to CreateSelect and constructed a SELECT statement
on the fly. It was a one-line call as opposed to the multiple lines I'd normally
have to write for a query. Here's the code:
1
2 {==========================================================
3 This function will create a SELECT or SELECT DISTINCT SQL
4 statement given input from a TStrings descendant like a
5 list. It will properly format the list into field decla-
6 rations of a SQL statement then, using the supplied
7 TableNm parameter, will construct an entire statement that
8 can be assigned to the SQL property of a TQuery.
9
10 Params: Distinct SELECT DISTINCT or regular SELECT
11 TableNm Table name: Should either be a fully
12 qualified table name, or preceeded by
13 an alias (ie, ':DbName:MyTable.db')
14 FieldList Any TStrings descendant will work here,
15 like the Items property of a TListBox.
16 ==========================================================}
17
18 function CreateSelect(Distinct: Boolean;
19 TableNm: string;
20 const FieldList: TStrings)
21 : TStrings;
22 var
23 Sql: TStringList;
24 I: Integer;
25 buf,
26 QueryType: string;
27 begin
28 //First, instantiate the SQL lines list
29 Sql := TStringList.Create;
30
31 //Determine whether or no this is a regular SELECT
32 //or a SELECT DISTINCT query.
33 if Distinct then
34 QueryType := 'SELECT '
35 else
36 QueryType := 'SELECT DISTINCT ';
37
38 buf := QueryType;
39
40 try
41 //Now add the fields to the select statement
42 //Notice that if we're on the last item,
43 //we don't want to add a trailing comma.
44 for I := 0 to (FieldList.Count - 1) do
45 if (I <> FieldList.Count - 1) then
46 buf := buf + FieldList[I] + ', '
47 else
48 buf := buf + FieldList[I];
49
50 //Now, put the query together
51 Sql.Add(buf);
52 Sql.Add('FROM "' + TableNm + '"');
53 Result := Sql;
54 finally
55 Sql.Free;
56 end;
57 end;
Here's a code snippet from one of my programs that implements the function:
58 var
59 qry: TQuery;
60 begin
61 qry := TQuery.Create(nil);
62 with qry do
63 begin
64 Active := False;
65 DatabaseName := 'MyLocalDatabase';
66 SQL := CreateSelect(True, ExtractFilePath(ParamStr(0))
67 + 'Process.DB', lbSelectedFlds.Items);
68 try
69 Open;
70 ....other code....
71 finally
72 Free;
73 end;
74 end;
75 end;
WHERE It Is....
I use a lot of TStrings types in my code because they make it easy to pass a bunch
of values at once. Especially with selection criteria in queries, having the
capability to select on multiple values is a boon to efficiency. The three
functions below can be added to a WHERE clause in a SQL statement for
multiple-value searching. All you need to do is assemble the search criteria into a
TStrings type like a TStringList or use even the Items property of a TListBox to be
able to use these functions. Respectively, they allow you to create a basic
multiple-value criteria statement (i.e. (fldName = 'thisvalue') OR (fldName =
'thatvalue'); an IN selection criteria statement; and a multiple-value LIKE
selection criteria statement. Here they are:
{Returns a '(FldName = FldValue) OR (FldName = FldValue)' etc string from
a list of values. Useful for translating TListBox values into SQL strings
IsValString is a boolean to test if the list values are a value string; that is,
it contains spaces, in which case, you would want double-quotes.}
76
77 function BuildSQLSetString(fldName: string; const List: TStrings;
78 IsValString: Boolean): string;
79 var
80 I: Integer;
81 buf: string;
82 begin
83 Result := '';
84 for I := 0 to (List.Count - 1) do
85 begin
86 if IsValString then
87 buf := '(' + fldName + ' = ''' + List[I] + ''') OR '
88 else
89 buf := '(' + fldName + ' = ' + List[I] + ') OR ';
90
91 Result := Result + buf;
92 end;
93 Result := Copy(Result, 1, Length(Result) - 4);
94 end;
95
96 //This will build an IN statement
97
98 function BuildSQLINString(fldName: string; const List: TStrings;
99 IsValString: Boolean): string;
100 var
101 I: Integer;
102 buf: string;
103 begin
104 Result := '';
105 for I := 0 to (List.Count - 1) do
106 if IsValString then
107 buf := buf + '''' + List[I] + ''', '
108 else
109 buf := buf + List[I] + ', ';
110
111 buf := Copy(buf, 1, Length(TrimRight(buf)) - 1);
112
113 Result := fldName + ' IN (' + buf + ')';
114 end;
115
116 //This will build a LIKE statement
117
118 function BuildSQLLikeString(fldName: string; const List: TStrings;
119 IsValString: Boolean): string;
120 var
121 I: Integer;
122 buf: string;
123 begin
124 Result := '';
125 for I := 0 to (List.Count - 1) do
126 begin
127 if IsValString then
128 buf := '(' + fldName + ' LIKE ''' + TrimRight(List[I]) + '%'') OR '
129 else
130 buf := '(' + fldName + ' LIKE ' + List[I] + '%) OR ';
131
132 Result := Result + buf;
133 end;
134 Result := Copy(Result, 1, Length(Result) - 4);
135 end;
Notice that in addition to the field name and value list, there's a parameter in
each function called IsValString. Set this to true if the field you're searching on
is a string field. Remember, discrete string values in SQL must be delimited by
single quotes.
Some MS SQL Server Stuff
A little over a year ago, I moved completely away from doing local table processing
with Paradox and started doing all my processing under a Client/Server environment
using MS SQL Server. I was in for a big surprise when I made the switch because
many of the things that I could do with Paradox tables that I took for granted,
weren't as easily done in SQL Server. Yikes! So, what I did was create some generic
routines specifically for Client/Server applications to make my life much easier.
Let's see what I've got...
How Many Was That?
For those of you who work with local tables, you know how easy it is to get the
number of records in a table: Simply connect a TTable object to the table in
question and query the RecordCount property. Well, in SQL Server, that's not so
easy because the idea of a "record" is non-existent in this set-based data
environment. But, just because you can't get the information from the table
directly, doesn't mean that it doesn't exist. In fact, the information is stored in
the system tables. Here's a function that will get you the number of rows contained
in a SQL Server table:
136
137 // =============================================================================
138 // Returns the SQL DB path of the specified BDE alias.
139 // =============================================================================
140
141 function GetSQLDBPath(const AliasName: ShortString): ShortString;
142 var
143 ParamList: TStrings; // List of parms from the BDE
144 Path: ShortString; // Path returned from the BDE
145 begin
146 Result := '';
147 ParamList := TStringList.Create;
148 try
149 Session.GetAliasParams(AliasName, ParamList);
150 Path := ParamList[0];
151 Result := Copy(Path, Pos('=', Path) + 1, Length(Path) - Pos('=', Path) + 1);
152 finally
153 ParamList.Free;
154 end;
155 end;
156
157 function GetSQLTableRows(SesName: string; DB: TDatabase; TableName: string):
158 Integer;
159 var
160 qry: TQuery;
161 begin
162 qry := TQuery.Create(nil);
163 with qry do
164 begin
165 Active := False;
166 SessionName := sesName;
167 DatabaseName := DB.DatabaseName;
168 SQL.Add('SELECT DISTINCT I.Rows');
169 SQL.Add('FROM ' + GetSQLDBPath(DB.DatabaseName) + '..SysIndexes AS I INNER
170 JOIN'
171 SQL.Add(' ' + GetSQLDBPath(DB.DatabaseName)
172 + '..SysObjects AS O ON (I.ID = O.ID)');
173 SQL.Add('WHERE (O.Type = ''U'') AND (O.Name = ''' + TableName + ''')');
174 try
175 Open;
176 Result := FieldByName('Rows').AsInteger;
177 finally
178 Free;
179 end;
180 end;
181 end;
Some of you might be wondering: "Of what use is this function?" Well, for some of
you, indeed, it might be of no use whatsoever. But in the type of applications I
write that query datasets with records numbering in the several millions, in some
steps, I only want to continue processing if my result sets aren't empty. This
function gives me a quick way of checking if I need to continue or not. So there!
To Transact or Not To Transact (SQL, that is)
If you're using SQL Server, but not using Transact SQL, you're missing out on a lot
of functionality. I know, I know, there are those cross-platform junkies out there
that will only write the most generic code so they can deploy their DB apps on
other servers. That's all well and good, but for the most of us, we only have a
single server platform, and to not use its native functionality is to deprive
ourselves of a lot of functionality. For instance, how many of you create temporary
tables when doing a series of queries? C'mon, raise yer hands high! Using strict
ANSI SQL with Delphi, you would have to create three objects: a TQuery to perform
the query, a TBatchMove to move the result table, and a TTable to receive the
results. Yikes! But with SQL Server, you can perform a SELECT INTO query use only a
single object: a TQuery. Check out the code snippet from one of my programs below:
182 var
183 qry: TQuery;
184 begin
185 qry := TQuery.Create(nil);
186 with qry do
187 begin
188 Active := False;
189 SessionName := ClassSession.SessionName;
190 DatabaseName := DBName;
191 SQL.Add('SELECT DISTINCT T.PatientID, T.Symptom);
192 SQL.Add('mp1);
193 SQL.Add('FROM ' + SymptomTable + ' T (TABLOCK), HoldingTable H (TABLOCK)');
194 SQL.Add('WHERE (H.PatientID = T.PatientID) AND (Age > 65) ');
195 SQL.Add('AND (Thiazides_Now IS NULL) AND (GOUT IS NULL)');
196 try
197 ExecSQL;
198 finally
199 Free;
200 end;
201 end;
202 // ....the rest of the code....
This simple query above accomplishes with one object using Transact-SQL that it
normally takes three objects to do with standard ANSI SQL. Notice that the query is
executed using ExecSQL as opposed to Open. If you're using Transact-SQL, everything
happens on the server, so you get no cursors back. That might present a problem in
some circumstances, but for the most part, since you're doing nothing but set
logic, having a cursor to a result set isn't necessary.
Not only does Transact-SQL make it shorter to do extracts, it has some cool syntax
that allows you to really short-circuit both code and performance. For instance,
one of the things that has always irked me about SQL is doing a two-table update;
that is, updating a field in a table from values contained in another table. In
Paradox, it's simple QBE query with matching example elements. In ANSI SQL, it
involves a subquery. Let's look at an example:
Standard ANSI SQL two-table update:
UPDATE ClaimsTbl
SET History = (SELECT Activity
FROM ActivityTbl AS A
WHERE (ClaimsTbl.Patient = A.Patient))
Not too complex, but let me tell you, it's as slow as molasses on SQL Server. On
the other hand, the following Transact-SQL UPDATE statement works lickety-split!
UPDATE ClaimsTbl
FROM ClaimsTbl C, Activity A
SET C.History = A.Activity
WHERE (C.Patient = A.Patient)
Not much different from the example above, but since SQL Server is equipped to deal
with this syntax in an optimized fashion, it works much faster.
Why have I spent so much time covering this stuff? Well, I'm a real proponent of
creating systems that run in the most optimized fashion I can make them. And that
means that I use all the tools available to me. Yes, it can be argued that I'm
locking myself into a specific platform's functionality, but since I only have one
platform that I'm dealing with, why not use it? I realize that I haven't covered
hardly any of the Transact-SQL syntax. If you've got it, I'll leave it up to you to
go hunting for the information (BTW, a great source is the SQL Server Books Online
reference that comes with the software). The bottom line is this: If you're
building systems that address a single server platform, make the best use of that
server's capabilities.
But Wait! There's More!
Below are some general purpose routines that I use on a regular basis for accessing
my SQL Server databases. Check 'em out:
203
204 //Uses Session.GetTableNames to get a list of tables from the specified database
205
206 procedure GetTableList(DBName: string; const TableList: TStrings);
207 var
208 db: TDatabase;
209 begin
210 db := TDatabase.Create(Application);
211 with db do
212 begin
213 DatabaseName := DBName;
214 LoginPrompt := False;
215 Connected := True;
216 end; { with }
217 try
218 Session.GetTableNames(db.DatabaseName, '', False, False, TableList);
219 db.Close;
220 finally
221 db.Free;
222 end;
223 end;
224
225 //Uses GetTableList to see if a particular table exists in database.
226
227 function TableExists(var ProcSession: TSession; var DB: TDatabase;
228 TableName: string): Boolean;
229 var
230 TableList: TStrings;
231 begin
232 Result := False;
233 TableList := TStringList.Create;
234 try
235 ProcSession.GetTableNames(DB.DatabaseName, '', False, False, TableList);
236 if (TableList.IndexOf(TableName) > 0) then
237 Result := True;
238 finally
239 TableList.Free;
240 end;
241 end;
242
243 //Performs a series of drops for all table names contained in the input array.
244 //Very useful for cleaning up a bunch of temporary tables at once.
245
246 procedure CleanMSSQLTbl(var ProcSession: TSession; DBName: string;
247 TableNames: array of string); overload;
248 var
249 sqlEI: TQuery;
250 I: Integer;
251 begin
252 for I := Low(TableNames) to High(TableNames) do
253 begin
254 sqlEI := TQuery.Create(nil);
255 with sqlEI do
256 begin
257 SessionName := ProcSession.SessionName;
258 DatabaseName := DBName;
259 with SQL do
260 begin
261 Add('if exists (select * from sysobjects where ' +
262 'id = object_id(''' + TableNames[I] + ''') and sysstat & 0xf = 3)');
263 Add('drop table ' + TableNames[I]);
264 try
265 ExecSQL;
266 finally
267 Free;
268 end;
269 end;
270 end;
271 end;
272 end;
273
274 //Will return a TStrings list containing the results of a query
275
276 function SQLLoadList(SesName, TblName, FldName: string): TStrings;
277 var
278 qry: TQuery;
279 begin
280 Result := TStringList.Create;
281 qry := TQuery.Create(nil);
282 with qry do
283 begin
284 Active := False;
285 if (SesName <> '') then
286 SessionName := SesName;
287 DatabaseName := ExtractFilePath(TblName);
288 SQL.Add('SELECT DISTINCT ' + FldName);
289 SQL.Add('FROM "' + TblName + '"');
290 try
291 Open;
292 while not EOF do
293 begin
294 Result.Add(FieldByName(FldName).AsString);
295 Next;
296 end;
297 finally
298 Free;
299 end;
300 end;
301 end;
Notice in most of the functions above, that they either require a TSession or a
TSession.SessionName as one of their formal parameters. This is because these
functions are thread-safe. In order to use these in a multi- threaded system, you
need to create a TSession instance for every thread, and database access within the
scope of the thread require a session name to operate under. Otherwise you'll get
access violations. Not good.
Well, that's it for now. I encourage you to use these functions and concepts in your own code. Believe me, they've saved me a lot of time!
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