SQL Prompt SQL syntax is a useful tool tips. It retrieves the database object name syntax, and code segments automatically, to provide users with the appropriate code for selection. Set automatic script makes the code easier to read - especially useful if the developer is not familiar with the script. SQL Prompt installation needed to use, can significantly improve the coding efficiency. In addition, users can customize as necessary to make it work in the intended manner.
This article explains the problem in adding non-empty row or column contains a null value to an existing table might encounter can change a non-empty. Demonstrate the safe deployment of such a change in the migration scripts.
In the design of the database to add or delete NULL constraints, which is a common ritual, but when you make changes to the table has been filled, there are some issues might make you feel the pain. When you try to add an unacceptable change NULL value of new or existing column can be empty columns as NOT NULL column, this situation may occur. If SQL Prompt detected will try to add a NOT NULL column to an existing table of the code, a warning is issued (EI028) to you, without specifying a default value.
I will demonstrate these questions, then show you how to develop applications for these types of changes build script. I'll show them how to work, whether you are starting from scratch to build a new version of the table, or change, or if you need to update an existing table so that it contains these changes.
NOT NULL added to the filled column table
We have a table, CountingWords where we record the old Welsh word for calculation. Our goal is to calculate to 20, but only know how to count to 10.
/* we create a table. Just for our example, we create the words
used to count from one to ten in old Welsh.
(Na, nid wyf yn siaradwr Cymraeg ond rwy'n hoffi gwneud
rhywfaint o ymchwil ieithyddol gyda'r nos)
I'd like to do from eleven to twenty as well eventually
so I'll add them and leave them NULL.
Here is the initial build script, with guard clauses
of course. We'll re-create every time it is run.
With a few modifications we can make it so it only runs once
which is safer if you have reckless colleagues in your shop.
*/
IF Object_Id('dbo.CountingWords') IS NOT NULL DROP TABLE dbo.CountingWords;
--we script version 1 of our table of counting words
CREATE TABLE dbo.CountingWords
(
TheValue INT NOT NULL,
Word NVARCHAR(30) NULL,
CONSTRAINT CountingWordsPK PRIMARY KEY (TheValue)
);
GO
INSERT INTO dbo.CountingWords (TheValue, Word)
VALUES
(1, 'Un'), (2, 'Dau'), (3, 'Tri'), (4, 'Pedwar'),
(5, 'Pump'), (6, 'Chwech'), (7, 'Saith'), (8, 'Wyth'),
(9, 'Naw'), (10, 'Deg'), (11, NULL), (12, NULL),
(13, NULL), (14, NULL), (15, NULL), (16, NULL),
(17, NULL), (18, NULL), (19, NULL), (20, NULL);
GO
Listing 1: Un, Dau, Tri - CountingWords table in the first edition
After the release of the first version of the table, we quickly realized that we really should record the name of the language, so we changed the design table to add a TheLanguage not accept NULLs in the column.
ALTER TABLE dbo.CountingWords ADD TheLanguage NVARCHAR(100) NOT NULL;
SQL Prompt立刻警告我们危险：
如果我们忽略SQL Prompt的警告并执行此操作，我们将会得到一个错误。
消息4901，级别16，状态1，行34
ALTER TABLE仅允许添加可以包含空值的列，或者指定了DEFAULT定义，或者添加的列是标识或时间戳列，或者如果没有以前的列条件满足表必须为空以允许添加此列。列'TheLanguage'不能添加到非空表'CountingWords'，因为它不满足这些条件。
错误消息是显式的，可以通过定义DEFAULT约束来轻松修复，以便SQL Server可以为每一行插入此新列的默认值。
ALTER TABLE dbo.CountingWords ADD TheLanguage NVARCHAR(100) NOT NULL DEFAULT 'Old Welsh';
清单2：添加NOT NULL列时指定默认值
简而言之，如果要添加列，并且不允许NULL值，则必须提供一个值以放入每一行。因为我们的表现在只有一种语言，“老威尔士语”，这不是太难。
当然，我们也想记录如何计算其他语言，例如Manx、Cornish或Cumbrian，因此为了强制执行一些数据完整性，我们需要创建一个名为“Location定义每种语言的父表” ，以及它们的最初记录。
让我们把它弄出来。对于这个表，我们不能只丢弃并重新创建没有数据丢失的表，一旦我们的CountingWords表通过FOREIGN KEY约束引用该表，我们就会得到一个错误。我们需要采取预防措施。如果重新运行代码，我将使用一种简单但相当奇怪的技术，确保在数据丢失方面没有损坏。此脚本必须作为多个批处理运行，因为CREATE TABLE语句必须位于批处理的开头，并且很难跨批处理有条件地执行代码。
IF Object_Id('dbo.Location') IS NOT NULL SET NOEXEC ON;
--cunning way of only executing a section
--of code on a condition. until the next SET NOEXEC OFF
--we script version 1 of our table of counting words
GO
--sadly the create table statement has to be at the start of a batch
CREATE TABLE dbo.Location
(
TheLanguage NVARCHAR(30) NOT NULL,
Description VARCHAR(100) NOT NULL DEFAULT '',
CONSTRAINT LanguageKey PRIMARY KEY (TheLanguage)
);
--now we insert the row we need for our existing data
INSERT INTO dbo.Location (TheLanguage) VALUES ('Old Welsh');
GO
SET NOEXEC OFF;
清单3：Location表的版本1
当然，现在我们还需要修改CountingWords表，使其TheLanguage列为a FOREIGN KEY，引用新Location表，但我们稍后会处理它。
更改可空列以使其不可为空
很快，我们决定允许列中的NULL值是我们想要解决的设计错误。我们已经了解到，如果表包含数据，SQL Server将不允许我们使列不可为空，除非我们为它提供默认值，在这种情况下只是一个空字符串。
ALTER TABLE CountingWords ADD CONSTRAINT WordConstraint DEFAULT '' FOR Word; ALTER TABLE CountingWords ALTER COLUMN Word NVARCHAR(30) NOT NULL;
消息515，级别16，状态2，行58
不能将值NULL插入“Word”列，表'PhilFactor.dbo.CountingWords'; 列不允许空值。更新失败。
该语句已终止。
清单4：尝试使Word列NOT NULL失败
AIEE！我们仍然无法使列不可为空，即使我们告诉SQL Server要为NULL列插入什么！首先，我们必须通过使用默认值更新所有行来显式删除任何现有的NULL值：
UPDATE CountingWords SET Word = DEFAULT WHERE Word IS NULL; ALTER TABLE CountingWords ALTER COLUMN Word NVARCHAR(30) NOT NULL;
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