2. HAVING.md

HAVING

Ushbu qo'llanmada siz guruh yoki agregat uchun qidiruv shartini belgilash uchun PostgreSQL HAVING bandidan qanday foydalanishni o'rganasiz.

HAVING bandi guruh yoki agregat uchun qidiruv shartini belgilaydi. HAVING bandi ko'pincha GROUP BY bandi bilan guruhlar yoki agregatlarni belgilangan shart asosida filtrlash uchun ishlatiladi.

Quyidagi bayonot HAVING bandini asosiy sintaksisini ko'rsatadi:

SELECT
	column1,
	aggregate_function (column2)
FROM
	table_name
GROUP BY
	column1
HAVING
	condition;

Ushbu sintaksisda band bo'yicha guruhlash column1 bo'yicha guruhlangan qatorlarni qaytaradi. HAVING bandi guruhlarni filtrlash shartini belgilaydi.

SELECT iborasining JOIN, LIMIT, FETCH kabi boshqa bandlarini qo'shish mumkin.

PostgreSQL HAVING bandini FROM, WHERE, GROUP BY dan keyin va SELECT, DISTINCT, ORDER BY va LIMIT bandlaridan oldin baholaydi.

HAVING bandi SELECT bandidan oldin baholanganligi sababli, HAVING bandida ustun taxalluslaridan foydalana olmaysiz. Chunki HAVING bandini baholash vaqtida SELECT bandida ko'rsatilgan ustun taxalluslari mavjud emas.

HAVING vs. WHERE

WHERE bandi belgilangan shart asosida qatorlarni filtrlash imkonini beradi. Biroq, HAVING bandi belgilangan shart bo'yicha qatorlar guruhlarini filtrlash imkonini beradi.

PostgreSQL HAVING bandiga misollar

Keling, namunaviy ma'lumotlar bazasidagi payment jadvalini ko'rib chiqaylik.

1. SUM() funktsiyasi misolida PostgreSQL HAVING bandidan foydalanish

Quyidagi so'rov har bir mijozning umumiy miqdorini topish uchun SUM() funksiyasi bilan GROUP BY bandidan foydalanadi:

SELECT
	customer_id,
	SUM (amount)
FROM
	payment
GROUP BY
	customer_id;

Quyidagi bayonotda 200 dan ortiq pul sarflagan yagona mijozlarni tanlash uchun HAVING bandi qo'shilgan:

SELECT
	customer_id,
	SUM (amount)
FROM
	payment
GROUP BY
	customer_id
HAVING
	SUM (amount) > 200;

2. COUNT() misol bilan PostgreSQL HAVING bandi

Namuna ma'lumotlar bazasidan quyidagi customer jadvaliga qarang:

Quyidagi so'rovda GROUP BY bandidan do'konga to'g'ri keladigan xaridorlar sonini topish uchun foydalaniladi:

SELECT
	store_id,
	COUNT (customer_id)
FROM
	customer
GROUP BY
	store_id

Quyidagi bayonotda 300 dan ortiq xaridorga ega bo'lgan do'konni tanlash uchun HAVING bandi qo'shiladi:

SELECT
	store_id,
	COUNT (customer_id)
FROM
	customer
GROUP BY
	store_id
HAVING
	COUNT (customer_id) > 300;

GROUP BY bandi tomonidan qaytarilgan guruh yoki agregat uchun qidiruv shartini belgilash uchun HAVING bandidan foydalaning.

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