IN vs EXISTS dalam SQL: Memahami Prestasi dan Penggunaan

IN vs EXISTS dalam MySQL: Contoh dan Penerangan Secara Hands-on

Dalam MySQL, IN dan EXISTS digunakan dalam pertanyaan untuk menapis data berdasarkan kehadiran baris dalam subkueri. Walau bagaimanapun, ia berfungsi dengan cara yang berbeza dan memilih antara mereka boleh memberi kesan kepada prestasi pertanyaan. Mari kita pecahkan perbezaan mereka dengan penjelasan dan contoh praktikal.

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1. DALAM Klausa

• Penerangan:
  Klausa IN digunakan untuk menapis baris berdasarkan sama ada nilai lajur sepadan dengan mana-mana nilai dalam senarai atau subkueri. Ia menyemak nilai padanan daripada pertanyaan dalam dan membandingkannya dengan pertanyaan luar.

• Prestasi:
  Klausa IN biasanya cekap apabila subkueri mengembalikan sejumlah kecil rekod. Walau bagaimanapun, jika subkueri mengembalikan set data yang besar, IN boleh menjadi lebih perlahan.

• Sintaks:
  

  SELECT columns 
  FROM table 
  WHERE column IN (subquery);

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2. Klausa WUJUD

• Penerangan:
  Klausa EXISTS menyemak kewujudan baris yang dikembalikan oleh subkueri. Jika subkueri mengembalikan sebarang baris, EXISTS menilai kepada TRUE dan pertanyaan luar diteruskan. Ia tidak mengambil berat tentang kandungan baris tetapi hanya sama ada baris itu wujud.

• Prestasi:
  EXISTS biasanya lebih pantas untuk set data yang besar kerana ia berhenti memproses sebaik sahaja ia menemui padanan. Ini menjadikannya cekap apabila bekerja dengan subkueri yang mengembalikan banyak baris.

• Sintaks:
  

  SELECT columns 
  FROM table 
  WHERE EXISTS (subquery);

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Contoh Hands-on

Mari pertimbangkan dua jadual: pelanggan dan pesanan.

Jadual pelanggan:

customer_id	customer_name
1	John Doe
2	Jane Smith
3	Alice Brown

Meja pesanan:

order_id	customer_id	order_total
1	1	200
2	1	150
3	2	300

We want to find all customers who have placed at least one order.

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Using the IN Clause

SELECT customer_name 
FROM customers 
WHERE customer_id IN (SELECT customer_id FROM orders);

Explanation:

• The subquery (SELECT customer_id FROM orders) returns all customer IDs that appear in the orders table.
• The outer query selects customers whose customer_id is in that result set.

Result:
| customer_name |
|---------------|
| John Doe |
| Jane Smith |

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Using the EXISTS Clause

SELECT customer_name 
FROM customers c
WHERE EXISTS (SELECT 1 FROM orders o WHERE o.customer_id = c.customer_id);

Explanation:

• The subquery SELECT 1 FROM orders o WHERE o.customer_id = c.customer_id checks whether any row in the orders table matches the customer_id of the current row from the customers table.
• If any match is found, EXISTS returns TRUE, and the customer is included in the result.

Result:
| customer_name |
|---------------|
| John Doe |
| Jane Smith |

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Key Differences

1. Return Values:

   • IN: Compares the values of a column with the result set of the subquery.
   • EXISTS: Returns TRUE or FALSE based on whether the subquery returns any rows.

2. Efficiency:

   • IN is more efficient for smaller datasets.
   • EXISTS is faster for large datasets, especially when the subquery returns many rows.

3. Use Case:

   • Use IN when you're comparing a column’s value against a small list of possible values.
   • Use EXISTS when you're checking for the presence of rows in a subquery (e.g., when there's a correlation between the outer and inner queries).

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Performance Example

Assume we have:

• 10,000 customers
• 100,000 orders

Query with IN:

SELECT customer_name 
FROM customers 
WHERE customer_id IN (SELECT customer_id FROM orders);

• Execution: MySQL will retrieve the entire result set from the subquery and compare it with each row in the outer query.

Query with EXISTS:

SELECT customer_name 
FROM customers c
WHERE EXISTS (SELECT 1 FROM orders o WHERE o.customer_id = c.customer_id);

• Execution: MySQL will check each row in the outer query and stop once it finds a matching row in the subquery, making it faster for large datasets.

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Conclusion

• Use IN when you have a simple list to compare or a small subquery result.
• Use EXISTS when you’re dealing with large datasets or need to check for the presence of related data in a subquery.

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