sql笔记

sql执行之步骤：

(8)  SELECT (9) DISTINCT (11) <TOP_specification> <select_list>
(1)  FROM <left_table>
(3)    <join_type> JOIN <right_table>
(2)      ON <join_condition>
(4)  WHERE <where_condition>
(5)  GROUP BY <group_by_list>
(6)  WITH {CUBE | ROLLUP}
(7)  HAVING <having_condition>
(10) ORDER BY <order_by_list>

详细说明：

1. 
FROM: A Cartesian product (cross join) is performed between the first two tables in the FROM clause, and as a result, virtual table VT1 is generated.

2. 
ON: The ON filter is applied to VT1. Only rows for which the <join_condition> is TRUE are inserted to VT2.

3. 
OUTER (join): If an OUTER JOIN is specified (as opposed to a CROSS JOIN or an INNER JOIN), rows from the preserved table or tables for which a match was not found are added to the rows from VT2 as outer rows, generating VT3. If more than two tables appear in the FROM clause, steps 1 through 3 are applied repeatedly between the result of the last join and the next table in the FROM clause until all tables are processed.

4. 
WHERE: The WHERE filter is applied to VT3. Only rows for which the <where_condition> is TRUE are inserted to VT4.

5. 
GROUP BY: The rows from VT4 are arranged in groups based on the column list specified in the GROUP BY clause. VT5 is generated.

6. 
CUBE | ROLLUP: Supergroups (groups of groups) are added to the rows from VT5, generating VT6.

7. 
HAVING: The HAVING filter is applied to VT6. Only groups for which the <having_condition> is TRUE are inserted to VT7.

8. 
SELECT: The SELECT list is processed, generating VT8.

9. 
DISTINCT: Duplicate rows are removed from VT8. VT9 is generated.

10. 
ORDER BY: The rows from VT9 are sorted according to the column list specified in the ORDER BY clause. A cursor is generated (VC10).

11. 
TOP: The specified number or percentage of rows is selected from the beginning of VC10. Table VT11 is generated and returned to the caller.