mysqlhaving用法，mysql CASE WHEN的基礎和多種用法

CASE
計算條件列表并返回多個可能結果表達式之一。

CASE 具有兩種格式：

簡單 CASE 函數將某個表達式與一組簡單表達式進行比較以確定結果。

mysqlhaving用法，
CASE 搜索函數計算一組布爾表達式以確定結果。?
兩種格式都支持可選的 ELSE 參數。

語法
簡單 CASE 函數：

CASE input_expression
??? WHEN when_expression THEN result_expression
??????? [ ...n ]
??? [?
??????? ELSE else_result_expression
??? END

CASE 搜索函數：

mysql使用教程？CASE
??? WHEN Boolean_expression THEN result_expression
??????? [ ...n ]
??? [?
??????? ELSE else_result_expression
??? END

參數
input_expression

是使用簡單 CASE 格式時所計算的表達式。Input_expression 是任何有效的 Microsoft? SQL Server? 表達式。

WHEN when_expression

mysql in。使用簡單 CASE 格式時 input_expression 所比較的簡單表達式。When_expression 是任意有效的 SQL Server 表達式。Input_expression 和每個 when_expression 的數據類型必須相同，或者是隱性轉換。

n

占位符，表明可以使用多個 WHEN when_expression THEN result_expression 子句或 WHEN Boolean_expression THEN result_expression 子句。

THEN result_expression

mysql主鍵？當 input_expression = when_expression 取值為 TRUE，或者 Boolean_expression 取值為 TRUE 時返回的表達式。result expression 是任意有效的 SQL Server 表達式。

ELSE else_result_expression

當比較運算取值不為 TRUE 時返回的表達式。如果省略此參數并且比較運算取值不為 TRUE，CASE 將返回 NULL 值。Else_result_expression 是任意有效的 SQL Server 表達式。Else_result_expression 和所有 result_expression 的數據類型必須相同，或者必須是隱性轉換。

WHEN Boolean_expression

mysql索引？使用 CASE 搜索格式時所計算的布爾表達式。Boolean_expression 是任意有效的布爾表達式。

結果類型
從 result_expressions 和可選 else_result_expression 的類型集合中返回最高的優先規則類型。有關更多信息，請參見數據類型的優先順序。

結果值
簡單 CASE 函數：
計算 input_expression，然后按指定順序對每個 WHEN 子句的 input_expression = when_expression 進行計算。

返回第一個取值為 TRUE 的 (input_expression = when_expression) 的 result_expression。

mysql是什么？
如果沒有取值為 TRUE 的 input_expression = when_expression，則當指定 ELSE 子句時 SQL Server 將返回 else_result_expression；若沒有指定 ELSE 子句，則返回 NULL 值。?
CASE 搜索函數：
按指定順序為每個 WHEN 子句的 Boolean_expression 求值。

返回第一個取值為 TRUE 的 Boolean_expression 的 result_expression。

如果沒有取值為 TRUE 的 Boolean_expression，則當指定 ELSE 子句時 SQL Server 將返回 else_result_expression；若沒有指定 ELSE 子句，則返回 NULL 值。?
示例
A. 使用帶有簡單 CASE 函數的 SELECT 語句
在 SELECT 語句中，簡單 CASE 函數僅檢查是否相等，而不進行其它比較。下面的示例使用 CASE 函數更改圖書分類顯示，以使其更易于理解。

?

sql case when 多條件，?

USE pubs
GO
SELECT??    Category = 
?????    CASE type
????????    WHEN 'popular_comp' THEN 'Popular Computing'
????????    WHEN 'mod_cook' THEN 'Modern Cooking'
????????    WHEN 'business' THEN 'Business'
????????    WHEN 'psychology' THEN 'Psychology'
????????    WHEN 'trad_cook' THEN 'Traditional Cooking'
????????    ELSE 'Not yet categorized'
?????    END,
??    CAST(title AS varchar(25)) AS 'Shortened Title',
??    price AS Price
FROM titles
WHERE price IS NOT NULL
ORDER BY type, price
COMPUTE AVG(price) BY type
GO

?

注釋，后來我試了一下不讓用category=

我的代碼是

sql中case when。SELECT??
? case gender
?WHEN 1 THEN 'NAN'
?WHEN 0 THEN 'NV'
end? as gender
FROM
t_swidy_day_nutrient

下面是結果集：

?

?

sql中casewhen的用法。Category??????????? Shortened Title?????????? Price?????????????????????
------------------- ------------------------- --------------------------
Business??????????? You Can Combat Computer S 2.99??????????????????????
Business??????????? Cooking with Computers: S 11.95?????????????????????
Business??????????? The Busy Executive's Data 19.99?????????????????????
Business??????????? Straight Talk About Compu 19.99?????????????????????

????????????????????????????????????????????? avg
????????????????????????????????????????????? ==========================
????????????????????????????????????????????? 13.73?????????????????????

Category??????????? Shortened Title?????????? Price?????????????????????
------------------- ------------------------- --------------------------
Modern Cooking????? The Gourmet Microwave???? 2.99??????????????????????
Modern Cooking????? Silicon Valley Gastronomi 19.99?????????????????????

????????????????????????????????????????????? avg
????????????????????????????????????????????? ==========================
????????????????????????????????????????????? 11.49?????????????????????

Category??????????? Shortened Title?????????? Price?????????????????????
------------------- ------------------------- --------------------------
Popular Computing?? Secrets of Silicon Valley 20.00?????????????????????
Popular Computing?? But Is It User Friendly? 22.95?????????????????????

????????????????????????????????????????????? avg
????????????????????????????????????????????? ==========================
????????????????????????????????????????????? 21.48?????????????????????

Category??????????? Shortened Title?????????? Price?????????????????????
------------------- ------------------------- --------------------------
Psychology????????? Life Without Fear???????? 7.00??????????????????????
Psychology????????? Emotional Security: A New 7.99??????????????????????
Psychology????????? Is Anger the Enemy??????? 10.95?????????????????????
Psychology????????? Prolonged Data Deprivatio 19.99?????????????????????
Psychology????????? Computer Phobic AND Non-P 21.59?????????????????????

????????????????????????????????????????????? avg
????????????????????????????????????????????? ==========================
????????????????????????????????????????????? 13.50?????????????????????

Category??????????? Shortened Title?????????? Price?????????????????????
------------------- ------------------------- --------------------------
Traditional Cooking Fifty Years in Buckingham 11.95?????????????????????
Traditional Cooking Sushi, Anyone???????????? 14.99?????????????????????
Traditional Cooking Onions, Leeks, and Garlic 20.95?????????????????????

????????????????????????????????????????????? avg
????????????????????????????????????????????? ==========================
????????????????????????????????????????????? 15.96?????????????????????

(21 row(s) affected)

?

B. 使用帶有簡單 CASE 函數和 CASE 搜索函數的 SELECT 語句
在 SELECT 語句中，CASE 搜索函數允許根據比較值在結果集內對值進行替換。下面的示例根據圖書的價格范圍將價格（money 列）顯示為文本注釋。

?

USE pubs
GO
SELECT???    'Price Category' = 
?????    CASE 
????????    WHEN price IS NULL THEN 'Not yet priced'
????????    WHEN price < 10 THEN 'Very Reasonable Title'
????????    WHEN price >= 10 and price < 20 THEN 'Coffee Table Title'
????????    ELSE 'Expensive book!'
?????    END,
??    CAST(title AS varchar(20)) AS 'Shortened Title'
FROM titles
ORDER BY price
GO

?

下面是結果集：

?

Price Category??????? Shortened Title?????
--------------------- --------------------
Not yet priced??????? Net Etiquette???????
Not yet priced??????? The Psychology of Co
Very Reasonable Title The Gourmet Microwav
Very Reasonable Title You Can Combat Compu
Very Reasonable Title Life Without Fear???
Very Reasonable Title Emotional Security:
Coffee Table Title??? Is Anger the Enemy?
Coffee Table Title??? Cooking with Compute
Coffee Table Title??? Fifty Years in Bucki
Coffee Table Title??? Sushi, Anyone???????
Coffee Table Title??? Prolonged Data Depri
Coffee Table Title??? Silicon Valley Gastr
Coffee Table Title??? Straight Talk About
Coffee Table Title??? The Busy Executive's
Expensive book!?????? Secrets of Silicon V
Expensive book!?????? Onions, Leeks, and G
Expensive book!?????? Computer Phobic And
Expensive book!?????? But Is It User Frien

(18 row(s) affected)

?

C. 使用帶有 SUBSTRING 和 SELECT 的 CASE 函數
下面的示例使用 CASE 和 THEN 生成一個有關作者、圖書標識號和每個作者所著圖書類型的列表。

?

USE pubs
SELECT SUBSTRING((RTRIM(a.au_fname) + ' '+ 
??    RTRIM(a.au_lname) + ' '), 1, 25) AS Name, a.au_id, ta.title_id,
??    Type = 
  CASE 
???    WHEN SUBSTRING(ta.title_id, 1, 2) = 'BU' THEN 'Business'
???    WHEN SUBSTRING(ta.title_id, 1, 2) = 'MC' THEN 'Modern Cooking'
???    WHEN SUBSTRING(ta.title_id, 1, 2) = 'PC' THEN 'Popular Computing'
???    WHEN SUBSTRING(ta.title_id, 1, 2) = 'PS' THEN 'Psychology'
???    WHEN SUBSTRING(ta.title_id, 1, 2) = 'TC' THEN 'Traditional Cooking'
  END
FROM titleauthor ta JOIN authors a ON ta.au_id = a.au_id

?

下面是結果集：

???

Name?????????????????????    au_id??????    title_id Type???????????????    
------------------------- ----------- -------- ------------------- 
Johnson White????????????    172-32-1176 PS3333??    Psychology?????????    
Marjorie Green???????????    213-46-8915 BU1032??    Business???????????    
Marjorie Green???????????    213-46-8915 BU2075??    Business???????????    
Cheryl Carson????????????    238-95-7766 PC1035??    Popular Computing??    
Michael O'Leary??????????    267-41-2394 BU1111??    Business???????????    
Michael O'Leary??????????    267-41-2394 TC7777??    Traditional Cooking 
Dean Straight????????????    274-80-9391 BU7832??    Business???????????    
Abraham Bennet???????????    409-56-7008 BU1032??    Business???????????    
Ann Dull?????????????????    427-17-2319 PC8888??    Popular Computing??    
Burt Gringlesby??????????    472-27-2349 TC7777??    Traditional Cooking 
Charlene Locksley????????    486-29-1786 PC9999??    Popular Computing??    
Charlene Locksley????????    486-29-1786 PS7777??    Psychology?????????    
Reginald Blotchet-Halls??    648-92-1872 TC4203??    Traditional Cooking 
Akiko Yokomoto???????????    672-71-3249 TC7777??    Traditional Cooking 
Innes del Castillo???????    712-45-1867 MC2222??    Modern Cooking?????    
Michel DeFrance??????????    722-51-5454 MC3021??    Modern Cooking?????    
Stearns MacFeather???????    724-80-9391 BU1111??    Business???????????    
Stearns MacFeather???????    724-80-9391 PS1372??    Psychology?????????    
Livia Karsen?????????????    756-30-7391 PS1372??    Psychology?????????    
Sylvia Panteley??????????    807-91-6654 TC3218??    Traditional Cooking 
Sheryl Hunter????????????    846-92-7186 PC8888??    Popular Computing??    
Anne Ringer??????????????    899-46-2035 MC3021??    Modern Cooking?????    
Anne Ringer??????????????    899-46-2035 PS2091??    Psychology?????????    
Albert Ringer????????????    998-72-3567 PS2091??    Psychology?????????    
Albert Ringer????????????    998-72-3567 PS2106??    Psychology??

(25 row(s) affected)

???

?

CASE 可能是 SQL 中被誤用最多的關鍵字之一。雖然你可能以前用過這個關鍵字來創建字段，但是它還具有更多用法。例如，你可以在 WHERE 子句中使用 CASE。

首先讓我們看一下 CASE 的語法。在一般的 SELECT 中，其語法如下：

SELECT <myColumnSpec> = 
CASE 
WHEN <A> THEN <somethingA> 
WHEN <B> THEN <somethingB> 
ELSE <somethingE> 
END

?

在上面的代碼中需要用具體的參數代替尖括號中的內容。下面是一個簡單的例子：

USE pubs 
GO 
SELECT Title, 'Price Range' = CASE WHEN price IS NULL THEN 'Unpriced' WHEN price < 10 THEN 'Bargain' WHEN price BETWEEN 10 and 20 THEN 'Average' ELSE 'Gift to impress relatives' END 
FROM titles 
ORDER BY price 
GO

?

這是 CASE 的典型用法，但是使用 CASE 其實可以做更多的事情。比方說下面的 GROUP BY 子句中的 CASE：

SELECT 'Number of Titles', Count(*) 
FROM titles 
GROUP BY CASE WHEN price IS NULL THEN 'Unpriced' WHEN price < 10 THEN 'Bargain' WHEN price BETWEEN 10 and 20 THEN 'Average' ELSE 'Gift to impress relatives' END 
GO

?

你甚至還可以組合這些選項，添加一個 ORDER BY 子句，如下所示：

USE pubs 
GO 
SELECT CASE WHEN price IS NULL THEN 'Unpriced' WHEN price < 10 THEN 'Bargain' WHEN price BETWEEN 10 and 20 THEN 'Average' ELSE 'Gift to impress relatives' END AS Range, Title 
FROM titles 
GROUP BY CASE WHEN price IS NULL THEN 'Unpriced' WHEN price < 10 THEN 'Bargain' WHEN price BETWEEN 10 and 20 THEN 'Average' ELSE 'Gift to impress relatives' END, Title 
ORDER BY CASE WHEN price IS NULL THEN 'Unpriced' WHEN price < 10 THEN 'Bargain' WHEN price BETWEEN 10 and 20 THEN 'Average' ELSE 'Gift to impress relatives' END, Title 
GO

?

注意，為了在 GROUP BY 塊中使用 CASE，查詢語句需要在 GROUP BY 塊中重復 SELECT 塊中的 CASE 塊。

除了選擇自定義字段之外，在很多情況下 CASE 都非常有用。再深入一步，你還可以得到你以前認為不可能得到的分組排序結果集。