这章应该是难点，也是成为SQL高手的必经之路。
注意有GROUP 语句时，WHERE和HAVING的场合。
前者用于检索前的条件过滤 。
后者用于检索出来结果之后的条件过滤。
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SELECT product_cd, MAX(avail_balance) max_balance, MIN(avail_balance) min_balance, AVG(avail_balance) avg_balance, SUM(avail_balance) tot_balance, COUNT(*) num_accounts FROM account GROUP BY product_cd ;
SELECT COUNT(DISTINCT open_emp_id);
SELECT MAX(pending_balance - avail_balance) max_uncleared FROM account;
SELECT COUNT(*) num_rows, COUNT(val) num_vals, SUM(val) total, MAX(val) max_val, AVG(val) avg_val FROM number_tbl;
SELECT product_cd, open_branch_id, SUM(avail_balance) total_balance FROM account GROUP BY product_cd, open_branch_id;
SELECT EXTRACT(YEAR FROM start_date) year, COUNT(*) how_many FROM employee GROUP BY EXTRACT(YEAR FROM start_date);
SELECT product_cd, open_branch_id, SUM(avail_balance) tot_balance FROM account GROUP BY product_cd, open_branch_id WITH ROLLUP;
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