Making Query Coding in SQL Easier by Implementing the SQL Divide Keyword - Advanced Database Query Systems - page 294

Example 1. A basic rewrite example
Input query:
(SELECT a, b
FROM T1)
DIVIDE
(SELECT b
FROM T2)
Rewritten query:
SELECT a
FROM T1
WHERE b IN
(SELECT b
FROM T2)
GROUP BY a
HAVING COUNT(a) = (SELECT COUNT(b)
FROM T2)
• Step 1: Are the brackets matched? Yes - This step checks that the number of left brackets equals
the number of right brackets.
• Step 2: Is the divide keyword present? Yes - If missing, no rewrite is performed. A trivial regular
expression checks for this keyword in any case.
• Step 3: Locate the position in the query where the first divide keyword occurs. Actually, the divide
keyword and its immediate surrounding brackets must be matched. This is to prevent a rogue table
with the word divide in it from damaging the rewrite.
(SELECT a, b
FROM T1)
DIVIDE
(SELECT b
FROM T2)
• Step 4: Scan leftward until the number of left brackets matches the number of right brackets from
the beginning of the divide keyword.
(SELECT a, b
FROM T1)
DIVIDE
(SELECT b
FROM T2)
• Step 5: Scan rightward until the number of right brackets matches the number of left brackets
from the end of the divide keyword.