union 与 union all 区别

union all 展示所有结果
union     展示去重后的结果

• union all

(SELECT
sc.IntelUserCode,
sc.StartDate,
sc.EndDate,
sc.DepartmentOfDuty,
sc.Duty,
sc.DutyStatus,
sc.BizType,
sc.TaskCode,
sc.ApproveStatus,
sc.Operator,
sc.Status,
bizname(sc.DepartmentOfDuty) AS DepartmentOfDutyName,
bizname(sc.Duty) AS Duty,
bizname(sc.DutyStatus) AS DutyStatus,
group_concat(ua.AttachmentCode) AS AttachmentCode
FROM StudentCadres sc
JOIN userattachment ua ON sc.IntelUserCode = ua.IntelUserCode AND ua.BizType = 'StudCadres'
WHERE
sc.Status = '1' AND sc.IntelUserCode = '7b9374f5-d21d-416e-bc1b-f632cd6542ee' AND ApproveStatus IN ('0', '3', '4')
GROUP BY
sc.IntelUserCode,
sc.StartDate,
sc.EndDate,
sc.DepartmentOfDuty,
sc.Duty,
sc.DutyStatus,
sc.BizType,
sc.TaskCode,
sc.ApproveStatus,
sc.Operator,
sc.Status)
UNION ALL
(SELECT
sc.IntelUserCode,
sc.StartDate,
sc.EndDate,
sc.DepartmentOfDuty,
sc.Duty,
sc.DutyStatus,
sc.BizType,
sc.TaskCode,
sc.ApproveStatus,
sc.Operator,
sc.Status,
bizname(sc.DepartmentOfDuty) AS DepartmentOfDutyName,
bizname(sc.Duty) AS Duty,
bizname(sc.DutyStatus) AS DutyStatus,
group_concat(ua.AttachmentCode) AS AttachmentCode
FROM un_StudentCadres sc
JOIN userattachment ua ON sc.IntelUserCode = ua.IntelUserCode AND ua.BizType = 'StudCadres'
WHERE sc.Status = '1' AND sc.IntelUserCode = '7b9374f5-d21d-416e-bc1b-f632cd6542ee' AND ApproveStatus IN ('1', '2')
GROUP BY
sc.IntelUserCode,
sc.StartDate,
sc.EndDate,
sc.DepartmentOfDuty,
sc.Duty,
sc.DutyStatus,
sc.BizType,
sc.TaskCode,
sc.ApproveStatus,
sc.Operator,
sc.Status)

Explain:

id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filterd	Extra
1	PRIMARY	sc		ref	IntelUserCode	IntelUserCode	108	const	1	5	Using index condition; Using where; Using filesort
1	PRIMARY	ua		ref	IntelUserCode	IntelUserCode	108	const	2	10	Using where; Using index
2	UNION	ua		ref	IntelUserCode	IntelUserCode	108	const	2	10	Using where; Using index; Using temporary; Using filesort
2	UNION	sc		ref	IntelUserCode	IntelUserCode	108	const	1	100	Using index condition; Using where

• union

(SELECT
sc.IntelUserCode,
sc.StartDate,
sc.EndDate,
sc.DepartmentOfDuty,
sc.Duty,
sc.DutyStatus,
sc.BizType,
sc.TaskCode,
sc.ApproveStatus,
sc.Operator,
sc.Status,
bizname(sc.DepartmentOfDuty) AS DepartmentOfDutyName,
bizname(sc.Duty) AS Duty,
bizname(sc.DutyStatus) AS DutyStatus,
group_concat(ua.AttachmentCode) AS AttachmentCode
FROM StudentCadres sc
JOIN userattachment ua ON sc.IntelUserCode = ua.IntelUserCode AND ua.BizType = 'StudCadres'
WHERE
sc.Status = '1' AND sc.IntelUserCode = '7b9374f5-d21d-416e-bc1b-f632cd6542ee' AND ApproveStatus IN ('0', '3', '4')
GROUP BY
sc.IntelUserCode,
sc.StartDate,
sc.EndDate,
sc.DepartmentOfDuty,
sc.Duty,
sc.DutyStatus,
sc.BizType,
sc.TaskCode,
sc.ApproveStatus,
sc.Operator,
sc.Status)
UNION
(SELECT
sc.IntelUserCode,
sc.StartDate,
sc.EndDate,
sc.DepartmentOfDuty,
sc.Duty,
sc.DutyStatus,
sc.BizType,
sc.TaskCode,
sc.ApproveStatus,
sc.Operator,
sc.Status,
bizname(sc.DepartmentOfDuty) AS DepartmentOfDutyName,
bizname(sc.Duty) AS Duty,
bizname(sc.DutyStatus) AS DutyStatus,
group_concat(ua.AttachmentCode) AS AttachmentCode
FROM un_StudentCadres sc
JOIN userattachment ua ON sc.IntelUserCode = ua.IntelUserCode AND ua.BizType = 'StudCadres'
WHERE sc.Status = '1' AND sc.IntelUserCode = '7b9374f5-d21d-416e-bc1b-f632cd6542ee' AND ApproveStatus IN ('1', '2')
GROUP BY
sc.IntelUserCode,
sc.StartDate,
sc.EndDate,
sc.DepartmentOfDuty,
sc.Duty,
sc.DutyStatus,
sc.BizType,
sc.TaskCode,
sc.ApproveStatus,
sc.Operator,
sc.Status)

Explain:

id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filterd	Extra
1	PRIMARY	sc		ref	IntelUserCode	IntelUserCode	108	const	1	5	Using index condition; Using where; Using filesort
1	PRIMARY	ua		ref	IntelUserCode	IntelUserCode	108	const	2	10	Using where; Using index
2	UNION	ua		ref	IntelUserCode	IntelUserCode	108	const	2	10	Using where; Using index; Using temporary; Using filesort
2	UNION	sc		ref	IntelUserCode	IntelUserCode	108	const	1	100	Using index condition; Using where
UNION RESULT			ALL							Using temporary

union 比 union all 多用了一个临时表存储去重后的数据