queriesUtilities.md

Declare Variables

-- regular
DECLARE @date DATETIME = '07.01.2019'

-- from select
DECLARE @count INT;
SET @count = (SELECT COUNT(sku) FROM product)
SELECT @count;

YYYY-MM date format

-- 2.5 million rows

-- 12 sec
select date
from sales

-- 13 sec
select concat(year(date), '-', RIGHT('00' + CONVERT(varchar(2), DATEPART(MONTH, date)), 2)) ym
from sales

-- 15 sec
select CAST(YEAR(date) AS VARCHAR(4)) + '-' + RIGHT('00' + CAST(MONTH(date) AS VARCHAR(2)), 2) as ym
from sales

-- 29 sec
select FORMAT(date, 'yyyy-MM') as ym
from sales

Only numbers

select barcode
from barcodes
where barcode not like '%[^0-9]%'

Remove trailing zeros

select cast(barcode as int)

Order a sub-query

Using top 99.9999 percent is a hack to achieve the ordering.

Note that top 100 percent won't work.

select
    sku,
    STRING_AGG(attribute, ', ') attributes
from (
    select top 99.9999999999999 percent
        sku,
        attribute
    from attributes
    order by
        sku asc,
        attribute asc
) t1
group by sku

Previous value

select
    lag(value) over (order by created) -- returns the previous row's value
from sales

CTE - Common Table Expression

It defines a temporary result set which can be used in a SELECT statement.

WITH
    salesrep AS (
        SELECT
            employeeNumber,
            CONCAT(firstName, ' ', lastName) AS salesrepName
        FROM
            employees
        WHERE
            jobTitle = 'Sales Rep'
    ),
    customer_salesrep AS (
        SELECT
            customerName, salesrepName
        FROM
            customers
                INNER JOIN
            salesrep ON employeeNumber = salesrepEmployeeNumber
    )
SELECT
    *
FROM
    customer_salesrep
ORDER BY customerName;