有条件地在php中管理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对其进行管理。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

这就是我想要的

$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，我想添加AND students_admission.admission_date = 'admission_date'仅当$ admission_date不为null时，此条件。所以，我尝试了这个

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。我该如何处理？

Answer 1

只需使用布尔逻辑：

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

您也可以这样表达：

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入查询字符串中：这既更高效也更安全（您的当前代码广泛暴露于SQL注入中）。有关原因和方式，请参见this famous SO post。

[另外，请注意，在查询的left join子句中在where ed表上放置条件实际上会将left join转换为inner join。您可能需要在on的join子句中使用此条件。