1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
|
<html>
<head>
<title>firstworks Programming with SQL Relay - Substitution and Bind Variables</title>
<link href="../css/styles.css" rel="stylesheet">
</head>
<body>
<span class="heading1">Substitution and Bind Variables</span><br><br>
<span class="heading2">What exactly are substitution and bind variables?</span><br><br>
<p>Substitution and input bind variables are both methods for replacing a
variable in a query or procedural code with a corresponding value from
your program. Some databases call bind variables parameters.</p>
<PRE>
<B><FONT color=#a62828>select</FONT></B>
first_name,
middle_initial,
last_name
<FONT color=#6959cf>from</FONT>
$(schema).people
<FONT color=#6959cf>where</FONT>
person_id=:id
<B><FONT color=#a62828>and</FONT></B>
age>=:youngage
<B><FONT color=#a62828>and</FONT></B>
age<=:oldage
</PRE>
<p>In this query, $(schema) is a substitution variable and :id, :youngage and
:oldage are input bind variables.</p>
<p>Output bind variables allow values to be passed from procedural code
into buffers in your program.</p>
<PRE>
<FONT color=#6959cf>BEGIN</FONT>
:returnval:=<FONT color=#ff00ff>100</FONT>*<FONT color=#ff00ff>50</FONT>;
<FONT color=#6959cf>END</FONT>;
</PRE>
<p>In this code, :returnval is an output bind variable.</p>
<p>Substitution variables are processed first, by the API. Input bind
variables are processed second, by the underlying database or by the
<b>SQL Relay</b> connection daemon in the event that the database doesn't
support bind variables. Output bind variables are processed by the database as
the query or procedural code is executed.</p>
<p>Input bind variables may appear as values in the WHERE clause of a SELECT,
UPDATE or DELETE, as values in the SET clause of an UPDATE, in the VALUES
clause of an INSERT or as values in a block of procedural code.</p>
<p>Output bind variables may appear in the RETURNING clause of a SELECT or as
variables in a block of procedural code.</p>
<p>Substitution variables may appear anywhere in the query. They are
frequently used to ammend WHERE clauses with additional constraints and specify
schemas or databases. A substitution value may even contain bind variables.</p>
<span class="heading2">Bind Variable Syntax</span><br><br>
<p>Different databases have different syntax for bind variables. Oracle bind
variables are names preceeded by a colon. In DB2 and Interbase, bind variables
are represented by question marks. In Sybase and MS SQL Server, bind variables
are names preceeded by an @ sign.</p>
<p>When using <b>SQL Relay</b> bind functions, to refer to an Oracle, Sybase
or MS SQL Server bind variable, you should use it's name without the preceeding
colon. To refer to a DB2 or Interbase bind variable, you should use it's
position number.</p>
<p>For example...</p>
<PRE>
<B><FONT color=#a62828>select</FONT></B>
first_name,
middle_initial,
last_name
<FONT color=#6959cf>from</FONT>
$(schema).people
<FONT color=#6959cf>where</FONT>
person_id=:id
<B><FONT color=#a62828>and</FONT></B>
age>=:youngage
<B><FONT color=#a62828>and</FONT></B>
age<=:oldage
</PRE>
<p>In this Oracle syntax query, you should use "id", "youngage" and "oldage"
as variable names in the inputBind functions.</p>
<PRE>
<B><FONT color=#a62828>select</FONT></B>
first_name,
middle_initial,
last_name
<FONT color=#6959cf>from</FONT>
$(schema).people
<FONT color=#6959cf>where</FONT>
person_id=?
<B><FONT color=#a62828>and</FONT></B>
age>=?
<B><FONT color=#a62828>and</FONT></B>
age<=?
</PRE>
<p>In this DB2 or Interbase syntax query, you should use "1", "2" and "3" as
variable names in the inputBind functions.</p>
<span class="heading2">Why should I use input bind variables instead of just
using substitution variables for everything?</span><br><br>
<p>Using input bind variables improves performance. A query can be prepared
once and executed multiple times, changing the bind variable values between
each execution.</p>
<p>Using bind variables improves the cache-hit rate for databases which cache
prepred queries as well. Databases which support bind variables parse the
query then plug input bind variables into the already parsed code. If the same
query is run a bunch of times, even with different values for the input bind
variables, the databse will have the code cached and won't have to parse
the query again. If you don't use input bind variables, the database will
parse the query each time because the where clause will be slightly
different each time and the code for all those slightly different
queries will clog the cache.</p>
<p>As a rule of thumb, you should use input bind variables instead of
substitutions in the WHERE clause of SELECT statements whenever you can.</p>
<p>Output bind variables allow values to be passed directly from procedural
code into buffers in your program. This is generally more convenient and
efficient than construcing a query that calls procedural code or constructing
procedural code that manufactures a result set.</p>
<span class="heading2">What if my database doesn't support bind variables?</span><br><br>
<p>The <b>SQL Relay</b> connection daemon will fake input binds for database
API's which don't natively support binds. Currently those are the mSQL, MySQL,
PostgreSQL, SQLite, FreeTDS and MDB Tools connections. You can use either
Oracle style or DB2/Interbase style bind variables with those databases, except
for FreeTDS which requires either Sybase/MS SQL Server style bind variables.
Output binds are not supported for these databases.</p>
<p>When using a database for which <b>SQL Relay</b> fakes bind variables, you
should make sure not to pass the wrong type of data into a bind variable. For
example, in the query:</p>
<PRE>
<B><FONT color=#a02828>select</FONT></B>
*
<FONT color=#6858c8>from</FONT>
testtable
<FONT color=#6858c8>where</FONT>
stringcol>:stringvar
</PRE>
<p>If stringcol is a string datatype such as char or varchar, bind a string to
stringvar rather than a long or double. Failure to do so will cause the
query to fail.</p>
</body>
</html>
|
