来点函数
    MAX()
    MIN()
    AVG()
    SUM()
    COUNT()

    选择invoice_total表内面值最大的发票
    SELECT MAX(invoice_total)
    FROM invoices

    SELECT
    MAX(invoice_total) AS highest,
    MIN(invoice_total) AS lowest,
    AVG(invoice_total) AS average,
    SUM(invoice_total) AS total,
    COUNT(invoice_total) AS number_of_invoices,
    COUNT(payment_date) AS count_of_payments, # 返回非空payment dates记录数
    COUNT(*) AS total_records
    FROM invoices

    选择下半年发票
    SELECT
    MAX(invoice_total) AS highest,
    MIN(invoice_total) AS lowest,
    AVG(invoice_total) AS average,
    SUM(invoice_total) AS total,
    COUNT(invoice_total) AS number_of_invoices,
    COUNT(payment_date) AS count_of_payments, # 返回非空payment dates记录数
    COUNT(*) AS total_records
    FROM invoices
    WHERE invoice_date > ‘2019-07-01’

    选择下半年发票，且client_id不重复
    SELECT
    MAX(invoice_total) AS highest,
    MIN(invoice_total) AS lowest,
    AVG(invoice_total) AS average,
    SUM(invoice_total) AS total,
    COUNT(invoice_total) AS number_of_invoices,
    COUNT(payment_date) AS count_of_payments, # 返回非空payment dates记录数
    COUNT(DISTINCT client_id) AS total_records
    FROM invoices
    WHERE invoice_date > ‘2019-07-01’

    SELECT
    SUM(invoice_total) AS total_sales,
    SUM(payment_total) AS total_payments,
    SUM(invoice_total - payment_total) AS what_we_except
    ‘First half of 2019’ AS date_range
    FROM invoices
    WHERE date
    BETWEEN ‘2019-01-01’ AND ‘2019-06-30’
    UNION
    SELECT
    SUM(invoice_total) AS total_sales,
    SUM(payment_total) AS total_payments,
    SUM(invoice_total - payment_total) AS what_we_except
    ‘Second half of 2019’ AS date_range
    FROM invoices
    UNION
    WHERE date
    BETWEEN ‘2019-07-01’ AND ‘2019-12-31’
    UNION
    SELECT
    SUM(invoice_total) AS total_sales,
    SUM(payment_total) AS total_payments,
    SUM(invoice_total - payment_total) AS what_we_except
    ‘total’ AS date_range
    FROM invoices
    WHERE date
    BETWEEN ‘2019-01-01’ AND ‘2019-12-31’