Bind variable vs. string-concatenated variable

Question

I've made an anonymous block that has dynamic SQL:

01 DECLARE
02 l_table_name VARCHAR2(100);
03 l_field_name VARCHAR2(100);
04 l_domain_name VARCHAR2(100);
05 
06 BEGIN 
07 FOR list_fields IN (
08 SELECT 
09 TABLE_NAME
10 ,FIELD_NAME
11 ,DOMAIN_NAME 
12 FROM 
13 ENG.FIELD_DOMAIN_ENG_VW 
14 WHERE 
15 TABLE_NAME NOT LIKE '%ANNO%' 
16 )
17 LOOP
18 l_table_name := list_fields.TABLE_NAME;
19 l_field_name := list_fields.FIELD_NAME;
20 l_domain_name := list_fields.DOMAIN_NAME;
21 
22 EXECUTE IMMEDIATE
23 'INSERT INTO ENG.CV_ERRORS
24 (TABLE_NAME, FIELD_NAME, ERROR)
25 SELECT
26 :bv1 AS TABLE_NAME
27 ,:bv2 AS FIELD_NAME --<<-- Lines in
28 , ' || l_field_name || ' AS ERROR --<<-- question.
29 FROM ' || 
30 l_table_name ||
31 ' LEFT JOIN
32 (
33 SELECT CODE
34 FROM ENG.D_CV_ENG_VW
35 WHERE DOMAIN = :bv3
36 )
37 ON ' || l_field_name || ' = CODE
40 WHERE
41 ' || l_field_name || ' IS NOT NULL
42 AND 
43 CODE IS NULL'
44 
45 USING l_table_name, l_field_name, l_domain_name;
46 
47 END LOOP;
48 COMMIT;
49 END;

---

+------------+------------+-------------+
| TABLE_NAME | FIELD_NAME | ERROR |
+------------+------------+-------------+
| TABLE_1 | FIELD_1 | BLACK SHEEP |
| TABLE_1 | FIELD_2 | zzzz |
| TABLE_2 | FIELD_1 | asdf |
+------------+------------+-------------+

---

There's a part of it that I don't understand how/why it works.

27 ,:bv2 AS FIELD_NAME
28 , ' || l_field_name || ' AS ERROR

Line 27 uses a bind variable that is tied to the l_field_name variable. The value that is returned is the name of the field (example: FIELD_1, FIELD_2, FIELD_1).

Line 28 breaks up the string, and uses the l_field_name variable directly (via concatenation). The value that is returned is the value of the field (example: BLACK SHEEP, zzzz, asdf).

Why do the bind variable (line 27) and the string-concatenated variable (line 28) return different values, when they reference the same thing (l_field_name)?

• miracle173: I dont know what you exactly want to know but it seems to be about bind variables. Is it really necessary to demonstrate your quesion that you show us such a complicated piece of code including a delete statement, an insert statement, a join, a subselect, a for loop. Isn't it possible to show us your problem using one table and one select?

Answer 1

You ask, referring to the variable l_field_name:

Why do the bind variable (line 27) and the string-concatenated variable (line 28) return different values, when they reference the same thing?

One way to look at this is to consider when and how the value of l_field_name is used.

The first (in terms of time) use is when you compose the string value that represents the INSERT statement. At that time the value of l_field_name is concatenated, along with other string literals and variable values, into the resultant string and becomes a part of it. The complete statement string is then passed to EXECUTE IMMEDIATE for parsing, compilation, and eventual execution.

The second use is when the value of l_field_name is assigned to the host (bind) variable :bv2 just prior to the statement execution. In this case that value is not concatenated or "injected" into the statement text: by this time the statement is already compiled into a plan, and :bv2 behaves like a variable it is -- it holds its value (that is now equal to the value of l_field_name) and passes it on to be inserted into the corresponding column.