我正在尝试制定一个具有 LEFT OUTER JOIN 的 UPDATE sql 语句。当 a.count 小于 subject._max_enrollment_no 或 a.count 为 NULL 时，应提交 UPDATE。当我尝试提交查询时，它会给出一个错误，如下所示。
UPDATE [e-request]
SET ereq_status = N'A'
FROM [e-request]
LEFT OUTER JOIN (SELECT Count(em_st_id) AS count,
em_sj_id
FROM enrollment
WHERE ( em_enrolled = 1 )
GROUP BY em_sj_id) AS a
ON [e-request.ereq_sj_id] = a.em_sj_id
RIGHT OUTER JOIN subjects
ON subjects.sj_id = [e-request.ereq_sj_id]
AND a.count < subjects.sj_max_enrollment_no
AND [e-request].ereq_id = @erequestID
应该如何更改查询以使其工作？架构显示在底部
USE [EnrolDB]
GO
/****** Object: Table [dbo].[enrollment] Script Date: 4/28/2013 6:14:58 PM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[enrollment](
[em_st_id] [int] NOT NULL,
[em_sj_id] [int] NOT NULL,
[em_order] [int] NOT NULL,
[em_semester] [int] NOT NULL,
[em_year] [int] NOT NULL,
[em_result] [float] NULL,
[em_enrolled] [bit] NOT NULL,
CONSTRAINT [PK__tmp_ms_x__76502B66F97E750E] PRIMARY KEY CLUSTERED
(
[em_st_id] ASC,
[em_sj_id] ASC,
[em_order] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
/****** Object: Table [dbo].[e-request] Script Date: 4/28/2013 6:14:59 PM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[e-request](
[ereq_id] [int] NOT NULL,
[ereq_st_id] [int] NOT NULL,
[ereq_sj_id] [int] NOT NULL,
[ereq_reason] [nvarchar](max) NOT NULL,
[ereq_status] [nvarchar](10) NOT NULL,
CONSTRAINT [PK_e-request] PRIMARY KEY CLUSTERED
(
[ereq_id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
GO
/****** Object: Table [dbo].[prerequisites] Script Date: 4/28/2013 6:14:59 PM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[prerequisites](
[pq_sj_id] [int] NOT NULL,
[pq_pq_sj_id] [int] NOT NULL,
CONSTRAINT [PK_prerequisites] PRIMARY KEY CLUSTERED
(
[pq_sj_id] ASC,
[pq_pq_sj_id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
/****** Object: Table [dbo].[student] Script Date: 4/28/2013 6:14:59 PM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[student](
[st_id] [int] NOT NULL,
[st_first_name] [nvarchar](50) NOT NULL,
[st_mid_name] [nvarchar](50) NULL,
[st_last_name] [nvarchar](50) NULL,
PRIMARY KEY CLUSTERED
(
[st_id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
/****** Object: Table [dbo].[subjects] Script Date: 4/28/2013 6:14:59 PM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[subjects](
[sj_id] [int] NOT NULL,
[sj_name] [nvarchar](64) NOT NULL,
[sJ_descr] [nvarchar](1024) NULL,
[sj_max_enrollment_no] [int] NULL,
[sj_avail] [bit] NULL,
PRIMARY KEY CLUSTERED
(
[sj_id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
/****** Object: Table [dbo].[Table] Script Date: 4/28/2013 6:14:59 PM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[Table](
[pq_sj_id] [int] NOT NULL,
[pq_pq_sj_id] [int] NOT NULL,
PRIMARY KEY CLUSTERED
(
[pq_sj_id] ASC,
[pq_pq_sj_id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
ALTER TABLE [dbo].[enrollment] WITH CHECK ADD CONSTRAINT [FK_enrollment_student] FOREIGN KEY([em_st_id])
REFERENCES [dbo].[student] ([st_id])
GO
ALTER TABLE [dbo].[enrollment] CHECK CONSTRAINT [FK_enrollment_student]
GO
ALTER TABLE [dbo].[enrollment] WITH CHECK ADD CONSTRAINT [FK_enrollment_subject] FOREIGN KEY([em_sj_id])
REFERENCES [dbo].[subjects] ([sj_id])
GO
ALTER TABLE [dbo].[enrollment] CHECK CONSTRAINT [FK_enrollment_subject]
GO
ALTER TABLE [dbo].[prerequisites] WITH CHECK ADD CONSTRAINT [FK_prerequisites_subjects] FOREIGN KEY([pq_sj_id])
REFERENCES [dbo].[subjects] ([sj_id])
GO
ALTER TABLE [dbo].[prerequisites] CHECK CONSTRAINT [FK_prerequisites_subjects]
GO
ALTER TABLE [dbo].[prerequisites] WITH CHECK ADD CONSTRAINT [FK_prerequisites_subjects2] FOREIGN KEY([pq_pq_sj_id])
REFERENCES [dbo].[subjects] ([sj_id])
GO
ALTER TABLE [dbo].[prerequisites] CHECK CONSTRAINT [FK_prerequisites_subjects2]
GO