如何比較mysql中的2個表

Question

我有兩個表名爲joy_gallery和joy_category。

• joy_gallery

  ╔════╦════════╦════════╗ 
  ║ id ║ cat_id ║ images ║ 
  ╠════╬════════╬════════╣ 
  ║ 1 ║ 1  ║ a.png ║ 
  ║ 2 ║ 1  ║ a1.png ║ 
  ║ 3 ║ 2  ║ b1.png ║ 
  ║ 4 ║ 1  ║ c.jpg ║ 
  ║ 5 ║ 2  ║ d.jng ║ 
  ║ 6 ║ 2  ║ a1.png ║ 
  ╚════╩════════╩════════╝ 
  

• joy_category

  ╔═════╦══════════╦═════════════╗ 
  ║ Cid ║ category ║ Description ║ 
  ╠═════╬══════════╬═════════════╣ 
  ║ 1 ║ onam  ║ xcfghhyjkk ║ 
  ║ 2 ║ xmas  ║ hjytigkuyy ║ 
  ╚═════╩══════════╩═════════════╝ 
  

我會向這兩個表進行比較，顯示兩大類，最後我像這樣的每個類別的法師，1 onam c.jpg和1 xmas a1.jpg。我正在使用這個mysql代碼：

SELECT * 
FROM joy_gallery INNER JOIN joy_category ON joy_gallery.cat_id = joy_category.Cid 
ORDER BY joy_category.Cid DESC; 

但它不工作。顯示來自同一類別的第一類和2個圖像。我該如何解決這個問題？如何顯示類別中的最後一張圖片？

Answer 1

有可能是這樣做的更有效的方法，但一個解決辦法是：

SELECT cat_id, images 
FROM 
    (SELECT MAX(galI.gaid) gaid 
    FROM joy_gallery galI INNER JOIN joy_category cat 
    ON galI.cat_id = cat.Cid GROUP BY cat_id) galIn 
INNER JOIN joy_gallery gal USING(gaid); 

Answer 2

試試這個：

SELECT j1.* FROM joy_gallery j1 
    JOIN (SELECT cid, MAX(cid) id FROM joy_category GROUP BY cid) j2 
    ON j1.cat_id = j2.cid ; 

Answer 3

您的查詢可能是這樣的：（tested）

SELECT joy_category.Cid, joy_category.category, joy_gallery.images FROM joy_gallery 
JOIN joy_category 
ON joy_gallery.cat_id = joy_category.Cid AND 
joy_gallery.id IN (
    SELECT MAX(joy_gallery.id) 
    FROM joy_gallery 
    GROUP BY joy_gallery.cat_id 
) 

輸出：

Cid category images 
---------------------------- 
    1  onam  c.jpg 
    2  xmas  a1.png 

Answer 4

嘗試此查詢

SELECT a.Cid, a.category, c.images 
FROM joy_category a 
LEFT JOIN (
    SELECT max(gaid) as id, cat_id 
    FROM joy_gallery 
    GROUP BY cat_id 
) b ON b.cat_id = a.Cid 
LEFT JOIN joy_gallery c ON c.gaid = b.id 

注意：這不是測試，但我認爲它的工作。

Answer 5

嘗試此查詢

SELECT 
    r.Cid, 
    r.category, 
    b.images 
FROM 
    joy_category b 
JOIN joy_gallery r ON r.Cid = b.cat_id 
AND r.id = (
    SELECT 
     MAX (x.id) 
    FROM 
     joy_gallery x 
    WHERE 
     x.cat_id = b.Cid 
);