Union standart so‘zidan foydalanish

Union standart so‘zidan foydalanish.
MySQL UNION operatori ikki yoki undan ortiq natijalar to'plamini bitta natijalar to'plamiga birlashtirishga imkon beradi. UNION operatorining sintaksisini quyidagilar aks ettiradi:
SELECT column_list
UNION [DISTINCT | ALL]
SELECT column_list
UNION [DISTINCT | ALL]
SELECT column_list
...
UNION operatorini qo’llash uchun quyidagi qoidalar mavjud:

• 
  Birinchidan, barcha SELECT operatorlarida paydo bo'ladigan ustunlar soni va tartiblari bir xil bo'lishi kerak.
  
• 
  Ikkinchidan, ustunlarning ma'lumotlar turlari bir xil yoki mos bo'lishi kerak.
  

Odatda UNION operatori takrorlanuvchi satrlarni o’chiradi. UNION operatorining Venn diagrammasi orqali ko’rinishi:

SELECT
firstName,
lastName
FROM
employees
UNION
SELECT
contactFirstName,
contactLastName
FROM
customers;

SELECT
CONCAT(firstName,' ',lastName) fullname

FROM
employees
UNION SELECT
CONCAT(contactFirstName,' ',contactLastName)
FROM
customers;

SELECT
CONCAT(firstName, ' ', lastName) fullname,

'Employee' as contactType
FROM
employees
UNION SELECT
CONCAT(contactFirstName, ' ', contactLastName),
'Customer' as contactType
FROM
customers
ORDER BY
fullname