Saralash, LIMIT va DISTINCT

Baza qatorlarni hech qanday kafolatlangan tartibsiz qaytaradi. Natijani tartiblash, birinchi bir nechtasini olish yoki takrorlarni olib tashlash uchun uchta buyruq bor.

ORDER BY — saralash

SELECT first_name, last_name, salary FROM teachers ORDER BY salary;

Odatda o‘sish tartibida (ASC) saralanadi. Kamayish tartibi uchun DESC:

SELECT first_name, last_name, salary FROM teachers ORDER BY salary DESC;

Bir nechta ustun bo‘yicha: avval birinchisi, teng bo‘lsa — ikkinchisi.

SELECT last_name, first_name, course FROM students ORDER BY course DESC, last_name;

LIMIT — faqat birinchi N ta qator

«Eng yuqori maoshli 3 ustoz»:

SELECT first_name, last_name, salary FROM teachers ORDER BY salary DESC LIMIT 3;

LIMIT doim ORDER BY bilan birga ishlatilsin — aks holda «qaysi 3 tasi» chiqishi tasodifga bog‘liq.

Keyingi «sahifa» uchun OFFSET bor: LIMIT 3 OFFSET 3 — 4-, 5-, 6-qatorlar.

DISTINCT — takrorsiz

Talabalar qaysi shaharlardan kelgan?

SELECT DISTINCT city FROM students ORDER BY city;

DISTINCT butun qatorga qo‘llanadi: SELECT DISTINCT city, course — shahar va kursning har xil juftliklari.

NULL qayerda turadi?

PostgreSQL saralashda NULL ni eng katta qiymat deb hisoblaydi: o‘sish tartibida u oxirida, DESC da boshida chiqadi. Buni o‘zgartirish mumkin: ORDER BY scholarship DESC NULLS LAST.

So‘rov qismlarining tartibi

Qismlar doim shu tartibda yoziladi:

SELECT … FROM … WHERE … ORDER BY … LIMIT …;

Esda qoldiring

• ORDER BY ustun [DESC] — saralash; bir nechta ustun vergul bilan.
• LIMIT n — birinchi n ta qator; doim ORDER BY bilan.
• DISTINCT — takrorlarni olib tashlaydi.