SELECT ec.id ,ec.title,ec.price ,ec.lesson_num ,ec.cover, ecd.description FROM `edu_course` ec LEFT JOIN edu_course_description ecd ON ec.id = ecd.id WHERE ec.id = #{courseId}，用union来代替left join写出具有相同作用的SQL语句

以下是使用 UNION 代替 LEFT JOIN 的 SQL 查询语句：

SELECT ec.id, ec.title, ec.price, ec.lesson_num, ec.cover, ecd.description
FROM edu_course ec, edu_course_description ecd
WHERE ec.id = ecd.id AND ec.id = #{courseId}
UNION
SELECT ec.id, ec.title, ec.price, ec.lesson_num, ec.cover, NULL AS description
FROM edu_course ec
WHERE ec.id = #{courseId} AND NOT EXISTS (SELECT 1 FROM edu_course_description ecd WHERE ec.id = ecd.id)

这条 SQL 语句首先使用 INNER JOIN 将 "edu_course" 表和 "edu_course_description" 表连接起来，获取有描述信息的课程的相关信息。然后使用 UNION 运算符将结果合并，并在第二个 SELECT 语句中使用 NOT EXISTS 子查询判断是否存在课程描述信息，如果不存在，则将 "description" 字段设置为 NULL。

相关问题

SELECT ec.id ,ec.title,ec.price ,ec.lesson_num ,ec.cover, ecd.description, et.`name`, es1.title as subjectLevelOne, es2.title as subjectLevelTwo FROM `edu_course` ec LEFT JOIN edu_course_description ecd ON ec.id = ecd.id LEFT JOIN edu_teacher et ON ec.teacher_id = et.id LEFT JOIN edu_subject es1 ON ec.subject_parent_id = es1.id LEFT JOIN edu_subject es2 ON ec.subject_id = es2.id WHERE ec.id = #{courseId}

这是一条 SQL 查询语句，目的是从 "edu_course" 表中获取指定 "courseId" 的课程信息及其对应的教师、科目等信息。具体查询的字段包括 "id"、"title"、"price"、"lesson_num"、"cover"、"description"、"name"、"subjectLevelOne"、"subjectLevelTwo"。同时，使用了 "LEFT JOIN" 连接多个表，将它们的相关信息进行关联。

帮我优化这段SQL：select count(*) as F_Add_Number from LAB_Order_Base base where 1=1 and base.F_Status_Word < 10000 and base.F_Status_Word > 1020 and base.F_ECD_Node3 >= '2023-05-17 00:00:00' and base.F_ECD_Node3 <= '2023-05-23 23:59:59' and (select top 1 rec.F_Order_Workflow_RecordID from LAB_Order_Workflow_Record rec where rec.F_Flow_Type = '0' and rec.F_Operation_Type = 'S' and rec.F_Flow_Node_Code = '2011' and rec.F_Order_NO = base.F_Order_NO) is not null

可以尝试以下优化方式：

1. 尽量避免使用子查询，可以使用 JOIN 操作来替代。如：

select count(*) as F_Add_Number  
from LAB_Order_Base base 
join LAB_Order_Workflow_Record rec on rec.F_Order_NO = base.F_Order_NO 
where base.F_Status_Word < 10000 and base.F_Status_Word > 1020  
and base.F_ECD_Node3 >= '2023-05-17 00:00:00' and base.F_ECD_Node3 <= '2023-05-23 23:59:59'  
and rec.F_Flow_Type = '0' and rec.F_Operation_Type = 'S' and rec.F_Flow_Node_Code = '2011'

2. 确保表有索引，可以创建索引来加速查询。对于该 SQL 语句，可以考虑在 LAB_Order_Base 表上创建如下联合索引：

CREATE INDEX idx_order_base ON LAB_Order_Base (F_Status_Word, F_ECD_Node3, F_Order_NO);

3. 建议使用 BETWEEN 来代替 >= 和 <= 的组合，如：

base.F_ECD_Node3 BETWEEN '2023-05-17 00:00:00' AND '2023-05-23 23:59:59'