有條件地管理 php 中的 mysql 查詢

Question

我有一個查詢，我在哪里獲取學生人數。 現在，我想修改該查詢並有條件地管理它。 這是我的查詢

SELECT count(students.id) AS student_count,
FROM students
LEFT JOIN students_admission ON students_admission.student_id = students.id
WHERE student_gender='Male'
AND students.address = 'Address'
GROUP BY id

這就是我要的

$admission_date = null;

SELECT count(students.id) AS student_count,
FROM students
LEFT JOIN students_admission ON students_admission.student_id = students.id
WHERE student_gender='Male'
AND students.address = 'Address'
AND students_admission.admission_date = '$admission_date'
GROUP BY id

但是，只有當 $admission_date 不是 null 時，我才想添加AND students_admission.admission_date = 'admission_date'這個條件。 所以，我試過這個

SELECT count(students.id) AS student_count,
FROM students
LEFT JOIN students_admission ON students_admission.student_id = students.id
WHERE student_gender='Male'
AND students.address = 'Address'
CASE WHEN '$admission_date' IS NOT NULL THEN students_admission.admission_date = '$admission_date' END
GROUP BY id

返回語法錯誤。 我該如何管理？

Answer 1

只需使用 boolean 邏輯：

AND (
    :admission_date IS NULL 
    OR students_admission.admission_date = :admission_date
)

您也可以將其表述為：

students_admission.admission_date = coalesce(:admission_date, students_admission.admission_date)

請注意，您應該使用參數化查詢而不是將字符串變量混合到查詢字符串中：這既更有效也更安全（您當前的代碼廣泛暴露於 SQL 注入）。 有關原因和方法，請參閱這篇著名的 SO 帖子。

另外，請注意，將條件放在查詢的where子句中的left join表上實際上會將left join變為inner join 。 您可能希望在join的on子句中使用此條件。