varchar to/from char

varchar to/from char
conversion varchar를 char로 변경할 경우 모든 컬럼타입을 동시에 변경해야 한다.
반대의 경우, 하나만 char->charchar변경시 다른 모든 컬럼도 varchar로 변경됨
참.. 특이하구만..

mysql> CREATE TABLE chartbl (name VARCHAR(40), address VARCHAR(80));
Query OK, 0 rows affected (0.05 sec)

mysql> desc chartbl;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| name    | varchar(40) | YES  |     | NULL    |       |
| address | varchar(80) | YES  |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+
2 rows in set (0.03 sec)

mysql> alter table chartbl modify name char(40);
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc chartbl;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| name    | varchar(40) | YES  |     | NULL    |       |
| address | varchar(80) | YES  |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> alter table chartbl modify name char(40), modify address char(80);
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc chartbl;
+---------+----------+------+-----+---------+-------+
| Field   | Type     | Null | Key | Default | Extra |
+---------+----------+------+-----+---------+-------+
| name    | char(40) | YES  |     | NULL    |       |
| address | char(80) | YES  |     | NULL    |       |
+---------+----------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql>


"For each article, find the dealer(s) with the most expensive price."

표준안
     SELECT article, dealer, price
     FROM   shop s1
     WHERE  price=(SELECT MAX(s2.price)
                   FROM shop s2
                   WHERE s1.article = s2.article);

수정안(최적화)
     CREATE TEMPORARY TABLE tmp (
             article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL,
             price   DOUBLE(16,2)             DEFAULT '0.00' NOT NULL);

     LOCK TABLES shop read;

     INSERT INTO tmp SELECT article, MAX(price) FROM shop GROUP BY article;

     SELECT shop.article, dealer, shop.price FROM shop, tmp
     WHERE shop.article=tmp.article AND shop.price=tmp.price;

     UNLOCK TABLES;

     DROP TABLE tmp;