Oracle 21c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
CREATE TABLE A (Subject, Marks) AS
SELECT 'A', 100 FROM DUAL UNION ALL
SELECT 'B', 200 FROM DUAL UNION ALL
SELECT 'C', 400 FROM DUAL UNION ALL
SELECT 'D', 500 FROM DUAL;
4 rows affected

CREATE TABLE B (Subject, Formula) AS
SELECT 'A', 'A/B' FROM DUAL UNION ALL
SELECT 'B', 'B/D' FROM DUAL UNION ALL
SELECT 'C', 'D/A-B' FROM DUAL UNION ALL
SELECT 'D', 'A+B/C-D' FROM DUAL;
4 rows affected

WITH shunting_yard (subject, formula, stack, output) AS (
  SELECT subject,
         formula,
         CAST(NULL AS VARCHAR2(4000)),
         CAST(NULL AS VARCHAR2(4000))
  FROM   B
UNION ALL
  SELECT subject,
         CASE
         WHEN SUBSTR(formula, 1, 1) NOT IN ('+', '-', '/', '*')
         THEN SUBSTR(formula, 2)
         WHEN stack IS NULL
         OR   DECODE(SUBSTR(stack, -1), '+', 1, '-', 1, '/', '2', '*', '2')
            < DECODE(SUBSTR(formula, 1, 1), '+', 1, '-', 1, '/', '2', '*', '2')
         THEN SUBSTR(formula, 2)
         ELSE formula
         END,
         CASE
         WHEN SUBSTR(formula, 1, 1) NOT IN ('+', '-', '/', '*')
         THEN stack
         WHEN stack IS NULL
         OR   DECODE(SUBSTR(stack, -1), '+', 1, '-', 1, '/', '2', '*', '2')
            < DECODE(SUBSTR(formula, 1, 1), '+', 1, '-', 1, '/', '2', '*', '2')
         THEN SUBSTR(formula, 1, 1) || stack
         ELSE SUBSTR(stack, 2)
         END,
         CASE
         WHEN SUBSTR(formula, 1, 1) NOT IN ('+', '-', '/', '*')
         THEN output || SUBSTR(formula, 1, 1)
         WHEN stack IS NULL
         OR   DECODE(SUBSTR(stack, -1), '+', 1, '-', 1, '/', '2', '*', '2')
            < DECODE(SUBSTR(formula, 1, 1), '+', 1, '-', 1, '/', '2', '*', '2')
         THEN output
         ELSE output || SUBSTR(stack, 1, 1)
         END
  FROM   shunting_yard
)
SEARCH DEPTH FIRST BY subject, formula SET order_id
CYCLE subject, formula, stack, output SET done TO 1 DEFAULT 0,
reverse_polish (subject, formula, stack, value) AS (
  SELECT subject,
         output,
         CAST(NULL AS VARCHAR2(4000)),
         CAST(NULL AS NUMBER)
  FROM   shunting_yard
  WHERE  done = 1
UNION ALL
  SELECT r.subject,
         SUBSTR(r.formula, 2),
         CASE
         WHEN SUBSTR(r.formula, 1, 1) IN ('+', '-', '*', '/')
         THEN '_' || SUBSTR(r.stack, 3)
         ELSE SUBSTR(r.formula, 1, 1) || r.stack
         END,
         CASE SUBSTR(r.formula, 1, 1)
         WHEN '+'
         THEN COALESCE(a2.marks, r.value) + COALESCE(a1.marks, r.value)
         WHEN '-'
         THEN COALESCE(a2.marks, r.value) - COALESCE(a1.marks, r.value)
         WHEN '/'
         THEN COALESCE(a2.marks, r.value) / COALESCE(a1.marks, r.value)
         WHEN '*'
         THEN COALESCE(a2.marks, r.value) * COALESCE(a1.marks, r.value)
         ELSE r.value
         END
  FROM   reverse_polish r
         LEFT OUTER JOIN a a1
         ON SUBSTR(r.stack, 1, 1) = a1.subject
         LEFT OUTER JOIN a a2
         ON SUBSTR(r.stack, 2, 1) = a2.subject
)
SEARCH DEPTH FIRST BY subject, formula SET order_id
CYCLE subject, formula, stack SET done TO 1 DEFAULT 0
SELECT subject, value
FROM   reverse_polish
WHERE  done = 1


SUBJECT
VALUE


A
.5

B
.4

C
-195

D
-399.5