爲什麼觸發器在使用'插入'表中的字段時嘗試插入NULL值？

Question

我必須將在MSSQL中完成的更改與遠程MySQL數據庫同步。要同步的更改將發票和用戶添加到系統中。遠程服務器不會總是可以訪問，所以我試圖建立一種日誌表來存儲在MSSQL中完成的更改。

這裏是一個完全正常觸發：

CREATE TRIGGER [dbo].[dokument_insert] 
    ON [dbo].[dokument] 
    AFTER INSERT 
AS 
BEGIN 
    SET NOCOUNT ON; 
    INSERT INTO [bcg_ekodu].[dbo].[sync_stack] (event,sql, table_name, import_priority) 
     SELECT 
      'INSERT', 
      'INSERT INTO bills SET 
       date = "'+CONVERT(VARCHAR(19),dok_kuup,120)+'", 
       total = "'+CAST(kokkusum AS nvarchar)+'", 
       number = "'+RTRIM(dok_nr)+'", 
       created = "'+CONVERT(VARCHAR(19),savetime,120)+'", 
       rounded = "'+CAST(ymardus AS nvarchar)+'", 
       currency = "'+CAST(valuuta AS nvarchar)+'", 
       due_date = "'+CONVERT(VARCHAR(19),tasupaev,120)+'", 
       pk_joosep = "'+CAST(dok_kood AS nvarchar)+'", 
       joosep_hankija = "'+CAST(hankija AS nvarchar)+'"; 
      UPDATE 
       bills, users, companies 
      SET 
       bills.user_id = users.id, 
       bills.imported = NOW() 
      WHERE 
       bills.imported IS NULL 
       AND companies.id = users.company_id 
       AND companies.pk_joosep = 10 
       AND bills.user_id = users.pk_joosep', 
      'bills', 
      '200' 
     FROM inserted 
END 

，每天一個行插入「庫門」表時插入行「sync_stack」表。 'sql'列將包含一個SQL，以在另一個（MySQL）數據庫中創建相同類型的行。

但這觸發不工作：

CREATE TRIGGER [dbo].[klient_insert] 
    ON [dbo].[klient] 
    AFTER INSERT 
AS 
BEGIN 
    SET NOCOUNT ON; 
    INSERT INTO [bcg_ekodu].[dbo].[sync_stack] (event,sql, table_name, import_priority) 
     SELECT 
      'INSERT', 
      'INSERT INTO users SET 
       username =10'+CAST(kl_kood as nvarchar)+', 
       password = NULL, 
       name ="'+LTRIM(RTRIM(kl_nimi))+'", 
       email ="'+CAST(LTRIM(RTRIM(kl_email)) as nvarchar)+'", 
       reference_no ="'+CAST(LTRIM(RTRIM(kl_viide)) as nvarchar)+'", 
       phone ="'+CAST(LTRIM(RTRIM(kl_tel1)) as nvarchar)+'", 
       logins ="'+CAST(0 as nvarchar)+'", 
       last_login = NULL, 
       created ="'+CONVERT(VARCHAR(19),savetime,120)+'", 
       updated = NULL, 
       deleted ="0", 
       address ="'+CAST(LTRIM(RTRIM(kl_aadr1)) as nvarchar)+'",  
       pk_joosep ="'+CAST(kl_kood as nvarchar)+'"', 
      'users', 
      '210' 
     FROM inserted 
END 

雖然上面的SQL創建觸發器執行完成就好了，當我嘗試了一些行插入到「觸發」表中，我得到的以下錯誤：

No row was updated. 

The data in row 175 was not committed. 
Error Source: .Net SqlClient Data Provider. 
Error Message: Cannot insert the value NULL into column 'sql', table 'mydb.dbo.sync_stack'; column does not allow nulls. INSERT fails. 

The statement has been terminated. 

Correct the errors and retry or press ESC to cancel the change(s). 

• 如果我刪除此觸發器，則不會出現此錯誤。
• 如果我只爲'sql'列插入純文本，它按預期工作。
• 如果我使用插入行中的任何字段，即使只是一個文本字段，它也會再次失敗。
• 如果我在'sql'列允許NULL值，插入行成功，但我'SQL'列中獲得NULL值。

如何使第二個觸發器按預期工作呢？

Answer 1

串聯爲NULL任何值爲NULL：

select 'test' + NULL 

結果無效，你應該使用類似的東西爲你列：

select isnull(column, '') 

這將導致一個空字符串。

Answer 2

我懷疑至少有一個插入到你的SQL語句中的值是NULL。您可以使用COALESCE（例如

username =10'+COALESCE(CAST(kl_kood as nvarchar), '')+', 

當然，你不應該聲明nvarchar沒有指定長度，對不對？

http://sqlblog.com/blogs/aaron_bertrand/archive/2009/10/09/bad-habits-to-kick-declaring-varchar-without-length.aspx