在 SQL 的 SELECT 語句中實(shí)現(xiàn) IF/ELSE 邏輯主要通過 CASE 表達(dá)式完成，1.CASE WHEN 結(jié)構(gòu)可根據(jù)條件返回不同值，如根據(jù)工資區(qū)間標(biāo)記 Low/Medium/High；2.MySQL 提供 IF() 函數(shù)用于簡單二選一判斷，如標(biāo)記是否符合獎金資格；3.CASE 可結(jié)合布爾表達(dá)式處理多條件組合，如判斷“高薪且年輕”的員工類別；總體而言，CASE 更靈活適用于復(fù)雜邏輯，IF 則適合簡化寫法。
在 SQL 的 SELECT 語句中使用 IF/ELSE 邏輯，主要是通過 CASE 表達(dá)式來實(shí)現(xiàn)的。雖然 SQL 沒有原生的 if/else 關(guān)鍵字可以直接寫在 SELECT 中，但 CASE 能夠非常靈活地完成類似的功能。
使用 CASE WHEN 實(shí)現(xiàn)條件判斷
SQL 中最常見的方法就是 CASE WHEN 結(jié)構(gòu)，它允許你在查詢結(jié)果中根據(jù)某些條件返回不同的值。
舉個例子：
SELECT name,
salary,
CASE
WHEN salary < 3000 THEN 'Low'
WHEN salary BETWEEN 3000 AND 7000 THEN 'Medium'
ELSE 'High'
END AS salary_level
FROM employees;這段代碼會根據(jù)員工工資的不同區(qū)間，在查詢結(jié)果里新增一個字段 salary_level，顯示為 Low、Medium 或 High。
這種方式非常適合在查詢時對數(shù)據(jù)做分類、標(biāo)記或者轉(zhuǎn)換。
在 SELECT 中嵌套 IF 判斷（MySQL 特有語法）
如果你用的是 MySQL，它提供了一個簡化的函數(shù)：IF()，可以在 SELECT 里直接使用。
語法是這樣的：
IF(condition, value_if_true, value_if_false)
例如：
SELECT name,
salary,
IF(salary > 5000, 'Eligible', 'Not Eligible') AS bonus_status
FROM employees;這個寫法比 CASE 更簡潔，但只適用于簡單的二選一判斷，復(fù)雜邏輯還是得靠 CASE。
使用布爾表達(dá)式結(jié)合 CASE 做更靈活處理
有時候你希望判斷多個字段之間的關(guān)系，或者組合多個條件，這時候 CASE 也能輕松應(yīng)對。
比如你想判斷員工是否“高薪且年輕”：
SELECT name,
age,
salary,
CASE
WHEN salary > 6000 AND age < 30 THEN 'Young and Well-paid'
WHEN salary > 6000 THEN 'Well-paid'
ELSE 'Others'
END AS category
FROM employees;這種結(jié)構(gòu)可以讓你在一個字段中體現(xiàn)出多種邏輯分支的結(jié)果，特別適合生成報(bào)表或數(shù)據(jù)標(biāo)簽。
基本上就這些。
在實(shí)際開發(fā)中，掌握 CASE WHEN 就能滿足大部分 SQL 查詢中的條件判斷需求。MySQL 的 IF() 可以作為簡化寫法偶爾使用。只要理解了邏輯結(jié)構(gòu)，寫起來并不難，但也很容易因?yàn)闂l件嵌套太深而變得難以維護(hù)，所以建議保持清晰的縮進(jìn)和注釋。
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