这段sql没有获取到结果，帮我优化它： CASE WHEN (( `a`.`EPI_MOCVD_STATUS_AT` IS NOT NULL ) AND isnull( `b`.`SURFACE_INSPECTION_STATUS_AT` )) THEN '外延生长' WHEN (( `b`.`SURFACE_INSPECTION_STATUS_AT` IS NOT NULL ) AND isnull( `c`.`PERFORMANCE_SAMPLING_STATUS_AT` )) THEN 'SURFACE检测' WHEN isnull( `a`.`EPI_MOCVD_STATUS_AT` ) THEN '性能抽检' END ) AS `WORK_STATION_NAME`

可以尝试优化这段 SQL 代码，将 CASE 语句转换为使用 COALESCE 函数来进行条件判断和返回结果。以下是优化后的代码：

COALESCE(
    CASE
        WHEN `a`.`EPI_MOCVD_STATUS_AT` IS NOT NULL AND ISNULL(`b`.`SURFACE_INSPECTION_STATUS_AT`) THEN '外延生长'
    END,
    CASE
        WHEN `b`.`SURFACE_INSPECTION_STATUS_AT` IS NOT NULL AND ISNULL(`c`.`PERFORMANCE_SAMPLING_STATUS_AT`) THEN 'SURFACE检测'
    END,
    CASE
        WHEN ISNULL(`a`.`EPI_MOCVD_STATUS_AT`) THEN '性能抽检'
    END
) AS `WORK_STATION_NAME`

通过使用 COALESCE 函数，可以避免多个嵌套的 WHEN 子句，提高代码的可读性和性能。

相关问题

数据库是mysql5.7，帮我优化下这条sql:SELECT epi.business_id,epi.project_id,epi.product_setmeal_id,count(1) num FROM emp_info ei LEFT JOIN (select epi.* from emp_project_info epi where epi.status = 1 AND epi.is_across = 0 AND epi.start_date = ( SELECT max( start_date ) FROM emp_project_info WHERE STATUS = 1 AND emp_id = epi.emp_id AND project_id = epi.project_id AND start_date <= '2023-05-01 00:30:00.028' and is_across = 0 ) AND epi.begin_date <= '2023-05-01 00:30:00.028' AND (epi.end_date is null or epi.end_date > '2023-05-01 00:30:00.028')) epi ON ei.id = epi.emp_id WHERE ei.status = 1 AND ei.type = 3 AND epi.project_id is not null AND epi.source != 5 GROUP BY epi.business_id,epi.project_id,epi.product_setmeal_id

为了优化这个 SQL，你可以考虑以下几点：

1. 将子查询用 JOIN 替代，以避免多层嵌套子查询的性能问题。

2. 为了避免子查询的重复计算，可以将子查询的结果保存到临时表中，并在 JOIN 时使用该临时表。

3. 需要为 emp_project_info 表添加合适的索引，以加快查询速度。

优化后的 SQL 如下：

先创建临时表：

CREATE TEMPORARY TABLE temp_ep_info AS
SELECT emp_id, project_id, product_setmeal_id, MAX(start_date) AS max_start_date
FROM emp_project_info
WHERE status = 1 AND is_across = 0 AND begin_date <= '2023-05-01 00:30:00.028' AND (end_date IS NULL OR end_date > '2023-05-01 00:30:00.028')
GROUP BY emp_id, project_id, product_setmeal_id;

然后执行查询：

SELECT epi.business_id, epi.project_id, epi.product_setmeal_id, COUNT(1) num
FROM emp_info ei
LEFT JOIN temp_ep_info tei ON ei.id = tei.emp_id
LEFT JOIN emp_project_info epi ON tei.emp_id = epi.emp_id AND tei.project_id = epi.project_id AND tei.product_setmeal_id = epi.product_setmeal_id AND tei.max_start_date = epi.start_date
WHERE ei.status = 1 AND ei.type = 3 AND epi.project_id IS NOT NULL AND epi.source != 5
GROUP BY epi.business_id, epi.project_id, epi.product_setmeal_id;

这样就可以避免多层嵌套子查询的性能问题，并且通过使用临时表来避免子查询的重复计算。同时，在 emp_project_info 表上添加合适的索引也可以提高查询速度。

帮我优化下这条sql:SELECT epi.business_id,epi.project_id,epi.product_setmeal_id,count(1) num FROM emp_info ei LEFT JOIN (select epi.* from emp_project_info epi where epi.status = 1 AND epi.is_across = 0 AND epi.start_date = ( SELECT max( start_date ) FROM emp_project_info WHERE STATUS = 1 AND emp_id = epi.emp_id AND project_id = epi.project_id AND start_date <= '2023-05-01 00:30:00.028' and is_across = 0 ) AND epi.begin_date <= '2023-05-01 00:30:00.028' AND (epi.end_date is null or epi.end_date > '2023-05-01 00:30:00.028')) epi ON ei.id = epi.emp_id WHERE ei.status = 1 AND ei.type = 3 AND epi.project_id is not null AND epi.source != 5 GROUP BY epi.business_id,epi.project_id,epi.product_setmeal_id

可以尝试以下优化：

1. 使用 EXISTS 替代子查询。

2. 将子查询中的 WHERE 条件移动到 JOIN 条件中。

3. 将比较运算符 "<=" 和 ">" 换成 BETWEEN。

4. 将 SELECT max(start_date) 改为 MAX(start_date) OVER(PARTITION BY emp_id, project_id)。

优化后的 SQL 如下：

SELECT epi.business_id, epi.project_id, epi.product_setmeal_id, COUNT(1) num
FROM emp_info ei
LEFT JOIN emp_project_info epi ON ei.id = epi.emp_id
WHERE ei.status = 1 AND ei.type = 3 AND epi.project_id IS NOT NULL AND epi.source != 5
AND epi.status = 1 AND epi.is_across = 0
AND epi.begin_date <= '2023-05-01 00:30:00.028'
AND (epi.end_date IS NULL OR epi.end_date > '2023-05-01 00:30:00.028')
AND epi.start_date BETWEEN MAX(epi.start_date) OVER(PARTITION BY epi.emp_id, epi.project_id) AND '2023-05-01 00:30:00.028'
GROUP BY epi.business_id, epi.project_id, epi.product_setmeal_id

这样可以避免子查询的计算，提高查询性能。