Avoiding Inner Capture in SELECT and DML Statements

Avoid inner capture of references in SELECT, SELECT INTO, and DML statements by following these recommendations:

Specify a unique alias for each table in the statement.
Do not specify a table alias that is the name of a schema that owns an item referenced in the statement.
Qualify each column reference in the statement with the appropriate table alias.

In Example B-7, schema hr owns tables tab1 and tab2. Table tab1 has a column named tab2, whose Abstract Data Type (ADT) has attribute a. Table tab2 does not have a column named a. Against recommendation, the query specifies alias hr for table tab1 and references table tab2. Therefore, in the query, the reference hr.tab2.a resolves to table tab1, column tab2, attribute a. Then the example adds column a to table tab2. Now the reference hr.tab2.a in the query resolves to schema hr, table tab2, column a. Column a of table tab2 captures the reference to attribute a in column tab2 of table tab1.

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Example B-7 Inner Capture of Attribute Reference

CREATE OR REPLACE TYPE type1 AS OBJECT (a NUMBER);
/
DROP TABLE tab1;
CREATE TABLE tab1 (tab2 type1);
INSERT INTO tab1 (tab2) VALUES (type1(10));

DROP TABLE tab2;
CREATE TABLE tab2 (x NUMBER);
INSERT INTO tab2 (x) VALUES (10);

/* Alias tab1 with same name as schema name,
   a bad practice used here for illustration purpose.
   Note lack of alias in second SELECT statement. */

SELECT * FROM tab1 hr
WHERE EXISTS (SELECT * FROM hr.tab2 WHERE x = hr.tab2.a);

Result:

TAB2(A)
---------------
 
TYPE1(10)
 
1 row selected.

Add a column named a to table tab2 (which belongs to schema hr):

ALTER TABLE tab2 ADD (a NUMBER);

Now, when the query runs, hr.tab2.a resolves to schema hr, table tab2, column a. To avoid this inner capture, apply the recommendations to the query:

SELECT * FROM hr.tab1 p1
WHERE EXISTS (SELECT * FROM hr.tab2 p2 WHERE p2.x = p1.tab2.a);

Qualifying References to Attributes and Methods

To reference an attribute or method of a table element, you must give the table an alias and use the alias to qualify the reference to the attribute or method.

In Example B-8, table tbl1 has column col1 of data type t1, an ADT with attribute x. The example shows several correct and incorrect references to tbl1.col1.x.

Example B-8 Qualifying ADT Attribute References

CREATE OR REPLACE TYPE t1 AS OBJECT (x NUMBER);
/
DROP TABLE tb1;
CREATE TABLE tb1 (col1 t1);

The references in the following INSERT statements do not need aliases, because they have no column lists:

BEGIN
  INSERT INTO tb1 VALUES ( t1(10) );
  INSERT INTO tb1 VALUES ( t1(20) );
  INSERT INTO tb1 VALUES ( t1(30) );
END;
/

The following references to the attribute x cause error ORA-00904:

UPDATE tb1 SET col1.x = 10 WHERE col1.x = 20;

UPDATE tb1 SET tb1.col1.x = 10 WHERE tb1.col1.x = 20;

UPDATE hr.tb1 SET hr.tb1.col1.x = 10 WHERE hr.tb1.col1.x = 20;

DELETE FROM tb1 WHERE tb1.col1.x = 10;

The following references to the attribute x, with table aliases, are correct:

UPDATE hr.tb1 t SET t.col1.x = 10 WHERE t.col1.x = 20;

DECLARE
  y NUMBER;
BEGIN
  SELECT t.col1.x INTO y FROM tb1 t WHERE t.col1.x = 30;
END;
/

DELETE FROM tb1 t WHERE t.col1.x = 10;

Qualifying References to Row Expressions

Row expressions must resolve as references to table aliases. A row expression can appear in the SET clause of an UPDATE statement or be the parameter of the SQL function REF or VALUE.

In Example B-9, table ot1 is a standalone nested table of elements of data type t1, an ADT with attribute x. The example shows several correct and incorrect references to row expressions.

Example B-9 Qualifying References to Row Expressions

CREATE OR REPLACE TYPE t1 AS OBJECT (x number);
/
DROP TABLE ot1;
CREATE TABLE ot1 OF t1;

BEGIN
  INSERT INTO ot1 VALUES (t1(10));
  INSERT INTO ot1 VALUES (20);
  INSERT INTO ot1 VALUES (30);
END;
/

The following references cause error ORA-00904:

UPDATE ot1 SET VALUE(ot1.x) = t1(20) WHERE VALUE(ot1.x) = t1(10);

DELETE FROM ot1 WHERE VALUE(ot1) = (t1(10));

The following references, with table aliases, are correct:

UPDATE ot1 o SET o = (t1(20)) WHERE o.x = 10;

DECLARE
  n_ref  REF t1;
BEGIN
  SELECT REF(o) INTO n_ref FROM ot1 o WHERE VALUE(o) = t1(30);
END;
/

DECLARE
  n t1;
BEGIN
  SELECT VALUE(o) INTO n FROM ot1 o WHERE VALUE(o) = t1(30);
END;
/

DECLARE
  n NUMBER;
BEGIN
  SELECT o.x INTO n FROM ot1 o WHERE o.x = 30;
END;
/

DELETE FROM ot1 o WHERE VALUE(o) = (t1(20));