有条件地管理 php 中的 mysql 查询

Question

我有一个查询，我在哪里获取学生人数。 现在，我想修改该查询并有条件地管理它。 这是我的查询

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

这就是我要的

$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，只有当 $admission_date 不是 null 时，我才想添加AND students_admission.admission_date = 'admission_date'这个条件。 所以，我试过这个

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

返回语法错误。 我该如何管理？

Answer 1

只需使用 boolean 逻辑：

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

您也可以将其表述为：

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

请注意，您应该使用参数化查询而不是将字符串变量混合到查询字符串中：这既更有效也更安全（您当前的代码广泛暴露于 SQL 注入）。 有关原因和方法，请参阅这篇著名的 SO 帖子。

另外，请注意，将条件放在查询的where子句中的left join表上实际上会将left join变为inner join 。 您可能希望在join的on子句中使用此条件。