Using EXISTS in SELECT

Question

In general,
If I want to construct a query, such as,
SELECT t1.fld1,t1.fld2,t1.fld3, mStr
FROM tbl t1
WHERE fld3 = 1
WHERE fld* are fields in table "tbl" and mStr is dynamic and is to hold a TRUE/FALSE value.
mStr is to be constructed using something like,
EXISTS ( SELECT * FROM tbl t2 where t2.fld2 = t1.fld2)
What's the correct syntax here?

Answer 1

I just realized that the code above will produce duplicate rows if there is more than one child, so DISTINCT should be added. The corrected code would look like this:
CREATE OR REPLACE PROCEDURE GetRecords
AS
CURSOR toplevel_cur
IS
SELECT DISTINCT b1.label,
b1.target,
b1.type,
b1.objectid,
DECODE (b2.parentid,
null, 'false ',
'true ') has_children
FROM buttons b1, buttons b2
WHERE b2.parentid(+) = b1.objectid
AND b1.lvl = 0;
BEGIN
FOR rec IN toplevel_cur
LOOP
DBMS_OUTPUT.PUT_LINE (rec.label
| | ' '
| | rec.target
| | ' '
| | rec.type
| | ' '
| | rec.objectid
| | ' '
| | rec.has_children);
END LOOP;
END GetRecords;
Also, if you are determined to use EXISTS, or just want to see that it produces the same results, you code use the code below, but I expect that it would run slower:
CREATE OR REPLACE PROCEDURE GetRecords
AS
CURSOR toplevel_cur
IS
SELECT b1.label,
b1.target,
b1.type,
b1.objectid,
'true ' has_children
FROM buttons b1
WHERE EXISTS
(SELECT b2.parentid
FROM buttons b2
WHERE b2.parentid = b1.objectid)
AND b1.lvl = 0
UNION
SELECT b1.label,
b1.target,
b1.type,
b1.objectid,
'false ' has_children
FROM buttons b1
WHERE NOT EXISTS
(SELECT b2.parentid
FROM buttons b2
WHERE b2.parentid = b1.objectid)
AND b1.lvl = 0;
BEGIN
FOR rec IN toplevel_cur
LOOP
DBMS_OUTPUT.PUT_LINE (rec.label
| | ' '
| | rec.target
| | ' '
| | rec.type
| | ' '
| | rec.objectid
| | ' '
| | rec.has_children);
END LOOP;
END GetRecords;
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