优化总结

CONCAT(str1,str2,…) 的使用

原代码片：

SELECT
    customer_name,
    customer_no,
    certificate_no,
    company_no,
    company_name,
    amount
FROM
    tb_loan_companies_customer
WHERE
    customer_name IN(
        SELECT
            customer_name
        FROM
            tb_loan_companies_customer
        GROUP BY
            customer_name,
            customer_no
        HAVING
            count(customer_name)> 1
        ORDER BY
            customer_name
    )
AND certificate_no IN(
    SELECT
        certificate_no
    FROM
        tb_loan_companies_customer
    GROUP BY
        customer_name,
        certificate_no
    HAVING
        count(certificate_no)> 1
    ORDER BY
        customer_name
)

经过优化的代码片：

SELECT
    customer_name,
    customer_no,
    certificate_no,
    company_no,
    company_name,
    amount
FROM
    tb_loan_companies_customer 
WHERE
    CONCAT(customer_name,customer_no,certificate_no) IN(
        SELECT CONCAT(customer_name,customer_no,certificate_no) 
        FROM tb_loan_companies_customer
        GROUP BY
            CONCAT(customer_name,customer_no,certificate_no)
        HAVING
            count(CONCAT(customer_name,customer_no,certificate_no))> 1
        ORDER BY
            CONCAT(customer_name,customer_no,certificate_no)
    )