CASE WHEN 詳解+實例

有時數據庫中爲了存儲空間的節約，存了一些比較小的字段，這是可以在select語句中進行轉換。

 

在select 語句中用條件語句：如果，數據庫中一個字段Gender來存儲性別，存的是1/0兩個值，這時可以用selectcasewhen gender=1 then'Man'else'Woman'endas [gender] from PersonInfo

 

SQL CASE 表達式是一種通用的條件表達式，類似於其它語言中的 if/else 語句。

CASE WHEN <tt><em>condition</em></tt> THEN <tt><em>result</em></tt>
     [WHEN ...]
     [ELSE <tt><em>result</em></tt>]
END

CASE 子句可以用於任何表達式可以有效存在的地方。 condition 是一個返回boolean 的表達式。 如果結果爲真，那麼 CASE 表達式的結果就是符合條件的 result。 如果結果爲假，那麼以相同方式搜尋任何隨後的 WHEN 子句。 如果沒有 WHEN condition 爲真，那麼 case 表達式的結果就是在 ELSE 子句裏的值。 如果省略了 ELSE 子句而且沒有匹配的條件， 結果爲 NULL。

例子：

SELECT * FROM test;

 a
---
 1
 2
 3


SELECT a,
       CASE WHEN a=1 THEN 'one'
	    WHEN a=2 THEN 'two'
	    ELSE 'other'
       END
    FROM test;

 a | case
---+-------
 1 | one
 2 | two
 3 | other

所有 result 表達式的數據的類型都必須可以轉換成單一的輸出類型。

下面這個"簡單的" CASE 表達式是上面的通用形式的一個特殊的變種。

CASE <tt><em>expression</em></tt>
    WHEN <tt><em>value</em></tt> THEN <tt><em>result</em></tt>
    [WHEN ...]
    [ELSE <tt><em>result</em></tt>]
END

先計算 expression 的值， 然後與所有在WHEN 子句裏聲明的 value 對比，直到找到一個相等的。 如果沒有找到匹配的，則返回在 ELSE 子句裏的 result （或者 NULL）。 這個類似於 C 裏的 switch 語句。

上面的例子可以用簡單 CASE 語法來寫：

SELECT a,
       CASE a WHEN 1 THEN 'one'
	      WHEN 2 THEN 'two'
	      ELSE 'other'
       END
    FROM test;

 a | case
---+-------
 1 | one
 2 | two
 3 | other

CASE 表達式並不計算任何對於判斷結果並不需要的子表達式。 比如，下面是一個可以避免被零除的方法：

SELECT ... WHERE CASE WHEN x <> 0 THEN y/x > 1.5 ELSE false END;

原文地址：http://blog.csdn.net/jiwenyi163/article/details/2794038

以下是case when 實例：

SELECT     dbo.FM_InHospital_Fee.EMPI_ID AS p_main_id, dbo.FM_InHospital_Fee.Inhosp_No AS p_settlement_code, dbo.FM_InHospital_Fee.Pat_Name AS p_name, 
                      dbo.PA_Patient_Information.Sex_Name AS p_sex, dbo.FM_InHospital_Fee.Inhosp_Index_No AS p_admit_id, 
                      dbo.PA_Inhospital_Registration.MR_No AS p_medical_record_id, dbo.FM_InHospital_Fee.Curr_Dept_Code AS p_admit_dept_code, 
                      dbo.FM_InHospital_Fee.Curr_Dept_Name AS p_admit_dept_name, dbo.PA_Inhospital_Registration.Admit_Bed_No AS p_admit_bed_no, 
                      dbo.PA_Inhospital_Registration.Admit_Date AS p_admit_date, dbo.PA_Inhospital_Registration.Discharge_Date AS p_discharge_date, '' AS p_admit_days, 
                      dbo.FM_InHospital_Fee.Medicare_Categ_Name AS p_medicare_type, dbo.PA_Patient_Information.HC_No AS p_social_security_id, 
                      dbo.FM_InHospital_Fee.Charge_Date AS p_settlement_date, dbo.FM_InHospital_Fee.Settlement_Staff_Name AS p_settlement_person, '' AS p_personal_fee, 
                      '' AS p_medicare_fund_payment, '' AS p_accept_fee, '' AS p_pay_fee, '' AS p_expect_fee, CASE WHEN Charge_Item_Code = 'BILL03' THEN 'Receivable_Fee' ELSE NULL
                       END AS p_western_medicine_fee, CASE WHEN Charge_Item_Code = 'BILL04' THEN 'Receivable_Fee' ELSE NULL END AS p_chinese_patent_medicine_fee, 
                      CASE WHEN Charge_Item_Code = 'BILL05' THEN 'Receivable_Fee' ELSE NULL END AS p_chinese_herbal_medicine_fee, 
                      CASE WHEN Charge_Item_Code = 'BILL06' THEN 'Receivable_Fee' ELSE NULL END AS p_inspect_fee, 
                      CASE WHEN Charge_Item_Code = 'BILL08' THEN 'Receivable_Fee' ELSE NULL END AS p_irradiation_fee, 
                      CASE WHEN Charge_Item_Code = 'BILL10' THEN 'Receivable_Fee' ELSE NULL END AS p_assay_fee, 
                      CASE WHEN Charge_Item_Code = 'BILL07' THEN 'Receivable_Fee' ELSE NULL END AS p_treatment_fee, 
                      CASE WHEN Charge_Item_Code = 'BILL09' THEN 'Receivable_Fee' ELSE NULL END AS p_operation_fee, 
                      CASE WHEN Charge_Item_Code = 'BILL11' THEN 'Receivable_Fee' ELSE NULL END AS p_oxygen_therapy_fee, 
                      CASE WHEN Charge_Item_Code = 'BILL01' THEN 'Receivable_Fee' ELSE NULL END AS p_medical_fee, 
                      CASE WHEN Charge_Item_Code = 'BILL02' THEN 'Receivable_Fee' ELSE NULL END AS p_bed_fee, 
                      CASE WHEN Charge_Item_Code = 'BILL12' THEN 'Receivable_Fee' ELSE NULL END AS p_other_fee, '' AS p_material_fee, '' AS p_intensive_care_fee, 
                      '' AS p_transfusion_fee, '' AS p_nurse_fee, dbo.FM_InHospital_Fee.Total_Fee AS p_add_fee
FROM         dbo.FM_InHospital_Fee INNER JOIN
                      dbo.PA_Patient_Information ON dbo.FM_InHospital_Fee.EMPI_ID = dbo.PA_Patient_Information.EMPI_ID INNER JOIN
                      dbo.PA_Inhospital_Registration ON dbo.FM_InHospital_Fee.EMPI_ID = dbo.PA_Inhospital_Registration.EMPI_ID