postgres —— 零碎笔记

-- 聚合查询
SELECT city, max(temp_lo)
FROM weather
WHERE city LIKE ‘S%‘
GROUP BY city
HAVING max(temp_lo) < 40; -- HAVING 子句始终包含聚合函数, 否则没有意义

-- 更新语句
UPDATE weather
    SET temp_hi = temp_hi - 2,  temp_lo = temp_lo - 2
    WHERE date > ‘1994-11-28‘;

-- 事务
BEGIN;
UPDATE accounts SET balance = balance - 100.00
    WHERE name = ‘Alice‘;
SAVEPOINT my_savepoint;
UPDATE accounts SET balance = balance + 100.00
    WHERE name = ‘Bob‘;
-- oops ... forget that and use Wally‘s account
ROLLBACK TO my_savepoint;
UPDATE accounts SET balance = balance + 100.00
    WHERE name = ‘Wally‘;
COMMIT;