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MySQL 的 CASE WHEN 语句 [复制链接]

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发表于 2011-04-11 11:30 |显示全部楼层
  1. /*

  2. mysql> select * from sales;
  3. +-----+------------+--------+--------+--------+------+------------+
  4. | num | name       | winter | spring | summer | fall | category   |
  5. +-----+------------+--------+--------+--------+------+------------+
  6. |   1 | Java       |   1067 |    200 |    150 |  267 | Holiday    |
  7. |   2 | C          |    970 |    770 |    531 |  486 | Profession |
  8. |   3 | JavaScript |     53 |     13 |     21 |  856 | Literary   |
  9. |   4 | SQL        |    782 |    357 |    168 |  250 | Profession |
  10. |   5 | Oracle     |    589 |    795 |    367 |  284 | Holiday    |
  11. |   6 | MySQL      |    953 |    582 |    336 |  489 | Literary   |
  12. |   7 | Cplus      |    752 |    657 |    259 |  478 | Literary   |
  13. |   8 | Python     |     67 |     23 |     83 |  543 | Holiday    |
  14. |   9 | PHP        |    673 |     48 |    625 |   52 | Profession |
  15. +-----+------------+--------+--------+--------+------+------------+
  16. 9 rows in set (0.01 sec)

  17. mysql> SELECT name AS Name,
  18.     -> CASE category
  19.     -> WHEN "Holiday" THEN "Seasonal"
  20.     -> WHEN "Profession" THEN "Bi_annual"
  21.     -> WHEN "Literary" THEN "Random" END AS "Pattern"
  22.     -> FROM sales;
  23. +------------+-----------+
  24. | Name       | Pattern   |
  25. +------------+-----------+
  26. | Java       | Seasonal  |
  27. | C          | Bi_annual |
  28. | JavaScript | Random    |
  29. | SQL        | Bi_annual |
  30. | Oracle     | Seasonal  |
  31. | MySQL      | Random    |
  32. | Cplus      | Random    |
  33. | Python     | Seasonal  |
  34. | PHP        | Bi_annual |
  35. +------------+-----------+
  36. 9 rows in set (0.00 sec)


  37. */
  38. Drop table sales;
  39.   
  40. CREATE TABLE sales(
  41.     num MEDIUMINT NOT NULL AUTO_INCREMENT,
  42.     name CHAR(20),
  43.     winter INT,
  44.     spring INT,
  45.     summer INT,
  46.     fall INT,
  47.     category CHAR(13),
  48.     primary key(num)
  49. )type=MyISAM;


  50. insert into sales value(1, 'Java', 1067 , 200, 150, 267,'Holiday');
  51. insert into sales value(2, 'C',970,770,531,486,'Profession');
  52. insert into sales value(3, 'JavaScript',53,13,21,856,'Literary');
  53. insert into sales value(4, 'SQL',782,357,168,250,'Profession');
  54. insert into sales value(5, 'Oracle',589,795,367,284,'Holiday');
  55. insert into sales value(6, 'MySQL',953,582,336,489,'Literary');
  56. insert into sales value(7, 'Cplus',752,657,259,478,'Literary');
  57. insert into sales value(8, 'Python',67,23,83,543,'Holiday');
  58. insert into sales value(9, 'PHP',673,48,625,52,'Profession');

  59. select * from sales;


  60. SELECT name AS Name,
  61. CASE category
  62. WHEN "Holiday" THEN "Seasonal"
  63. WHEN "Profession" THEN "Bi_annual"
  64. WHEN "Literary" THEN "Random" END AS "Pattern"
  65. FROM sales;
  66.            
复制代码
a1.jpg

[代码] 在SELECT查询中使用CASE WHEN
  1. /*
  2. mysql> SELECT Name, RatingID AS Rating,
  3.     ->    CASE RatingID
  4.     ->       WHEN 'R' THEN 'Under 17 requires an adult.'
  5.     ->       WHEN 'X' THEN 'No one 17 and under.'
  6.     ->       WHEN 'NR' THEN 'Use discretion when renting.'
  7.     ->       ELSE 'OK to rent to minors.'
  8.     ->    END AS Policy
  9.     -> FROM DVDs
  10.     -> ORDER BY Name;
  11. +-----------+--------+------------------------------+
  12. | Name      | Rating | Policy                       |
  13. +-----------+--------+------------------------------+
  14. | Africa    | PG     | OK to rent to minors.        |
  15. | Amadeus   | PG     | OK to rent to minors.        |
  16. | Christmas | NR     | Use discretion when renting. |
  17. | Doc       | G      | OK to rent to minors.        |
  18. | Falcon    | NR     | Use discretion when renting. |
  19. | Mash      | R      | Under 17 requires an adult.  |
  20. | Show      | NR     | Use discretion when renting. |
  21. | View      | NR     | Use discretion when renting. |
  22. +-----------+--------+------------------------------+
  23. 8 rows in set (0.01 sec)


  24. */

  25. Drop table DVDs;

  26. CREATE TABLE DVDs (
  27.    ID SMALLINT NOT NULL AUTO_INCREMENT PRIMARY KEY,
  28.    Name VARCHAR(60) NOT NULL,
  29.    NumDisks TINYINT NOT NULL DEFAULT 1,
  30.    RatingID VARCHAR(4) NOT NULL,
  31.    StatID CHAR(3) NOT NULL
  32. )
  33. ENGINE=INNODB;

  34. INSERT INTO DVDs (Name, NumDisks, RatingID, StatID)
  35. VALUES ('Christmas', 1, 'NR', 's1'),
  36.        ('Doc',       1, 'G',  's2'),
  37.        ('Africa',    1, 'PG', 's1'),
  38.        ('Falcon',    1, 'NR', 's2'),
  39.        ('Amadeus',   1, 'PG', 's2'),
  40.        ('Show',      2, 'NR', 's2'),
  41.        ('View',      1, 'NR', 's1'),
  42.        ('Mash',      2, 'R',  's2');
  43.   

  44. SELECT Name, RatingID AS Rating,
  45.    CASE RatingID
  46.       WHEN 'R' THEN 'Under 17 requires an adult.'
  47.       WHEN 'X' THEN 'No one 17 and under.'
  48.       WHEN 'NR' THEN 'Use discretion when renting.'
  49.       ELSE 'OK to rent to minors.'
  50.    END AS Policy
  51. FROM DVDs
  52. ORDER BY Name;
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