Analisis kes pertanyaan berbilang jadual MySQL

Pertanyaan berbilang jadual

Perihalan kes

Pemahaman produk Cartesian

select id,department_name
from employees,departments;#错的

select id,department_id,department_name
from employees CROSS JOIN departments;#错的

Setiap pekerja dan setiap jabatan dipadankan sekali (bilangan penyertaan ditemui = Bilangan id * bilangan jabatan)

Sebab ralat: Keadaan sambungan hilang

Penyelesaian kepada produk Cartesian

Tulis syarat sambungan: Jadual 1. Lajur = Jadual 2. Lajur (jika Untuk menyambung berbilang jadual, sekurang-kurangnya n-1 syarat sambungan mesti digunakan)

select id,employees.name,department_name 
from employees,departments
WHERE employees.name = departments.name;

Nota: Jika lajur yang akan dipaparkan mempunyai nama yang sama dalam jadual untuk disoal, ia mesti menunjukkan jadual asalnya, cth: pekerja .name

Adalah disyorkan untuk menunjukkan maklumat jadual yang dipaparkan apabila menanyakan berbilang jadual (pengoptimuman)

Pengoptimuman: Anda boleh menggunakan alias jadual selepas FROM, tetapi setelah alias digunakan, seterusnya Pastikan anda menggunakan alias

Klasifikasi pertanyaan berbilang jadual

Gabungan setara dan tidak setara

• Caburan setara: yang di atas dengan =

• Bukan equijoin: tidak =

select t1.id,t1.name,t2.grade
from employees t1,departments t2
WHERE ti.salary BETWEEN t2.lowest_salary AND t2.highest_salary ;#非等值

Self-join dan non-self-join

• Pautan bukan diri: Jadual 1 dan jadual 2 disambungkan

• Pautan diri: Jadual 1 dan dirinya sendiri disambungkan

#显示员工（t1）和其管理者（t2）的基本信息
select t1.id,t1.name,t2.id，t2.name
from employees t1,employees t2#一个表看作两个表
WHERE t1.manage_id = t2.id ;#自连接

Sambungan dalam dan sambungan luar Sertai

• Sambungan dalam: Gabungkan jadual yang mengandungi lajur yang sama Hasilnya tidak termasuk baris dalam satu jadual yang tidak sepadan dengan jadual lain

  < . : sambung luar kiri (lebih banyak jadual kiri, tambah kanan), sambung luar kanan (lebih banyak jadual kanan, isikan kiri), sambung luar penuh
• SQL92: Gunakan (+) untuk membuat sambungan
  Sambungan dalam: lihat di atas

WHERE t1.department_id = t2.department_id(+)#左连接

select t1.id,t1.name,t2.department_name,t3.environment
from employees t1 JOIN departments t2
ON t1.department_id = t2.department_id
JOIN locations t3#加入第二个人表
ON t2.department_location = t3.department_location;

​​

• PENUH LUAR JOIN: LUAR PENUH JOIN (MySQL tidak menyokong)

• select t1.name,t2.department_name#左外连接
  from employees t1 LEFT OUTER（可省略） JOIN departments t2
  ON t1.department_id = t2.department_id；

  Salin selepas log masuk
  Penggunaan UMION
• Gabung hasil pertanyaan

  SELECT colum... FROM table1
  UNION (ALL)
  SELECT colum... FROM table2

  Salin selepas log masuk

• UNION ALL operator (disyorkan)

• Pelaksanaan 7 jenis SQL JOINS

Gambar tengah (gabungan dalam):

select t1.name,t2.department_name
from employees t1 JOIN departments t2
ON t1.department_id = t2.department_id；

Gambar kiri atas (sambungan luar kiri) ):

select t1.name,t2.department_name
from employees t1 LEFT JOIN departments t2
ON t1.department_id = t2.department_id；

Gambar kanan atas (sambungan luar kanan):

select t1.name,t2.department_name
from employees t1 RIGHT JOIN departments t2
ON t1.department_id = t2.department_id；

select t1.name,t2.department_name
from employees t1 LEFT JOIN departments t2
ON t1.department_id = t2.department_id
WHERE t2.department_id IS NULL;

select t1.name,t2.department_name
from employees t1 RIGHT JOIN departments t2
ON t1.department_id = t2.department_id
WHERE t1.department_id IS NULL;

#方式一：左上图 UNION ALL 右中图
select t1.name,t2.department_name
from employees t1 LEFT JOIN departments t2
ON t1.department_id = t2.department_id
UNION ALL 
select t1.name,t2.department_name
from employees t1 RIGHT JOIN departments t2
ON t1.department_id = t2.department_id
WHERE t1.department_id IS NULL;

#方式二：左中图 UNION ALL 右上图
select t1.name,t2.department_name
from employees t1 LEFT JOIN departments t2
ON t1.department_id = t2.department_id
WHERE t2.department_id IS NULL
UNION ALL
select t1.name,t2.department_name
from employees t1 RIGHT JOIN departments t2
ON t1.department_id = t2.department_id；

#左中图 UNION ALL 右中图
select t1.name,t2.department_name
from employees t1 LEFT JOIN departments t2
ON t1.department_id = t2.department_id
WHERE t2.department_id IS NULL
UNION ALL
select t1.name,t2.department_name
from employees t1 RIGHT JOIN departments t2
ON t1.department_id = t2.department_id
WHERE t1.department_id IS NULL;

select t1.name,t2.department_name
from employees t1 JOIN departments t2
ON t1.department_id = t2.department_id;
等价于
select t1.name,t2.department_name
from employees t1 JOIN departments t2
USING(department_id);

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