Please explain the sql

Question

Hi All,
If anybody explain me briefly about the following Update statements
Thanks for you rhelp
1)UPDATE MRCH.MRC_TEMP_PAY A
     SET OFFSET = (SELECT MIN(NEW_PAY) FROM MRCH.MRC_TEMP_PAY B
WHERE A.PROJECT_ID = B.PROJECT_ID
AND A.MUNI = B.MUNI
AND B.NEW_PAY < -1
AND PAY_IND = 'N')
     WHERE A.NEW_PAY > 100
     AND A.PAY_IND = 'Y'
     AND A.PINSUB = (SELECT MIN(PINSUB) FROM MRCH.MRC_TEMP_PAY B
     WHERE A.PROJECT_ID = B.PROJECT_ID
     AND A.MUNI = B.MUNI
     AND B.NEW_PAY = (SELECT MAX(NEW_PAY) FROM MRCH.MRC_TEMP_PAY C
     WHERE C.PROJECT_ID = B.PROJECT_ID
     AND C.MUNI = B.MUNI
     AND C.NEW_PAY > 0
     AND C.PAY_IND = 'Y'));
-- NOW GET OFFSET PHASE
2)UPDATE MRCH.MRC_TEMP_PAY A
SET OFFSET_PH = (SELECT PHASE FROM MRCH.MRC_TEMP_PAY B
WHERE A.PROJECT_ID = B.PROJECT_ID
AND A.MUNI = B.MUNI
AND A.OFFSET = B.NEW_PAY)
WHERE A.PAY_IND = 'Y'
AND OFFSET IS NOT NULL;
-- CALCULATE NEW PAYMENT ADDING OFFSET
3) UPDATE MRCH.MRC_TEMP_PAY
     SET NEW_PAY = NEW_PAY + OFFSET
     WHERE OFFSET IS NOT NULL
     AND PAY_IND = 'Y';
     COMMIT;
     -- CHECK ANOTHER PHASE FOR OFFSET
4)UPDATE MRCH.MRC_TEMP_PAY A
     SET OFFSET2 = (SELECT MIN(NEW_PAY) FROM MRCH.MRC_TEMP_PAY B
WHERE A.PROJECT_ID = B.PROJECT_ID
AND A.MUNI = B.MUNI
AND B.NEW_PAY < -1
AND B.NEW_PAY != A.OFFSET
AND B.PAY_IND = 'N')
     WHERE A.NEW_PAY > 100
     AND A.PAY_IND = 'Y'
     AND A.PINSUB = (SELECT MIN(PINSUB) FROM MRCH.MRC_TEMP_PAY B
     WHERE A.PROJECT_ID = B.PROJECT_ID
     AND A.MUNI = B.MUNI
     AND B.NEW_PAY = (SELECT MAX(NEW_PAY) FROM MRCH.MRC_TEMP_PAY C
     WHERE C.PROJECT_ID = B.PROJECT_ID
     AND C.MUNI = B.MUNI
     AND C.NEW_PAY > 0
     AND C.PAY_IND = 'Y'))
     AND A.OFFSET IS NOT NULL;
     5)UPDATE MRCH.MRC_TEMP_PAY A
     SET OFFSET_PH2 = (SELECT PHASE FROM MRCH.MRC_TEMP_PAY B
WHERE A.PROJECT_ID = B.PROJECT_ID
AND A.MUNI = B.MUNI
AND A.OFFSET2 = B.NEW_PAY)
     WHERE A.NEW_PAY > 100
     AND A.PAY_IND = 'Y'
     AND A.OFFSET2 IS NOT NULL;
6)     UPDATE MRCH.MRC_TEMP_PAY
     SET NEW_PAY = NEW_PAY + OFFSET2
     WHERE OFFSET2 IS NOT NULL
     AND PAY_IND = 'Y';

Answer 1

Second query:
If the pay_ind column is yes and offset is not null then the offset_ph is set to the value returned by the inner query. The inner query returns the values of phase.
This is an example of correlated query. So the inner query is executed once for every row in the outer query.
For more information on correlated queries refer to:
http://download-west.oracle.com/docs/cd/B10501_01/server.920/a96540/statements_103a.htm#2066913
Third query:
This query adds the offset value to the pay if pay_id is yes and the offset is not null.
fifth query:
Same as third
The first and the fourth queries can be explained better if the names of the columns and the scenario are known.
Both are using correlated subqueries which are very costly.
Regards,
Anupama