请问下用SQL语句为表添加约束可不可用用判断

Posted 2023-02-20

在数据库中 ，在用SQL语句创建库和创建表之前可以用if exists来判断
但是用if exists在添加约束之前不行，请问添加约束之前可bu可以判断。
如果可以 请告诉一下方法 谢谢!!!

参考技术A SQL语言，数据库基础电子资料：

SQL语法大全中文版
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SQL语言案头完全参考手册
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SQL - A Practical Introduction
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O'Reilly SQL Tuning
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O'Reilly The Art of SQL
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数据库综合资料库
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数据库设计指南
http://bbs.topsage.com/dispbbs.asp?boardID=122&ID=148082
Wrox Beginning Database Design
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SQL Puzzles and Answers
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SQL Queries for Mere Mortals
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SQL Puzzles and Answers
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Apress出版 The Berkeley DB Book
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The Handbook of Data Mining
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数据库系统概论 浙江大学张军教授主讲(全32讲)
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MS SQL Server电子资料：

Transact-SQL Cookbook
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SQL Server 2005宝典
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Microsoft SQL Server 2005完全参考
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O'Reilly Learning SQL on SQL Server 2005
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Beginning SQL Server 2005 Programming
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Pro SQL Server 2005 High Availability
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Beginning SQL Server 2005 Administration
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SQL Server 2005 Management and Administration
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SQL Server 2005 Unleashed
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Pro SQL Server 2005
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A Developer's Guide to SQL Server 2005
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Pro T-SQL 2005 Programmer's Guide
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Beginning Transact-SQL with SQL Server 2000 and 2005
http://bbs.topsage.com/dispbbs.asp?boardID=122&ID=152868
SQL Server 2005报表服务
http://bbs.topsage.com/dispbbs.asp?boardID=122&ID=148376
Wrox Professional SQL Server 2005 Programming
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Scaling Out SQL Server 2005权威指南
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Sql Server 2005 Performance Optimiztion and Tuning Handbood
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Microsoft SQL Server 2005编程傻瓜书
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Pro SQL Server 2005 Assemblies
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MS SQL Server 2005 Reporting Essentials
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SQL Server 2005工具箱内幕
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SQL Server 2005工具箱内幕
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Pro SQL Server 2005 Service Broker
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Pro SQL Server 2005 Replication
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Sql server 2005的XML最佳实施策略
http://bbs.topsage.com/dispbbs.asp?boardID=122&ID=152278
Microsoft SQL Server Black Book
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MS SQL Server2000 宝典
http://bbs.topsage.com/dispbbs.asp?boardID=122&ID=145853
SQL Server 2000存储过程和XML编程
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SQL Server 2005高级数据分析视频教程系列
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SQL Server 2005盛宴系列视频 全52讲
http://bbs.topsage.com/dispbbs.asp?boardID=122&ID=151642

mysql电子资料：

MySQL宝典
http://bbs.topsage.com/dispbbs_122_159157_1.html
SQL for MySQL Developers
http://bbs.topsage.com/dispbbs.asp?boardID=122&ID=152862
MySQL教程
http://bbs.topsage.com/dispbbs.asp?boardID=122&ID=146824
Teach Yourself MySQL in 10 Minutes
http://bbs.topsage.com/dispbbs_122_174432_1.html
O'Reilly MySQL Pocket Reference 第二版 PDF
http://bbs.topsage.com/dispbbs.asp?boardID=122&ID=175014
MySQL 5 权威指南(第三版)
http://bbs.topsage.com/dispbbs.asp?boardID=122&ID=148867
MySQL培训经典教程
http://bbs.topsage.com/dispbbs.asp?boardID=122&ID=146795
MySQL Cookbook 第二版
http://bbs.topsage.com/dispbbs.asp?boardID=122&ID=175972
The Definitive Guide to MySQL
http://bbs.topsage.com/dispbbs.asp?boardID=122&ID=175382
O'Reilly MySQL Pocket Reference 第二版
http://bbs.topsage.com/dispbbs.asp?boardID=122&ID=175014
MySQL Essential Skills
http://bbs.topsage.com/dispbbs.asp?boardID=122&ID=146749
MySQL Administrators Guide
http://bbs.topsage.com/dispbbs.asp?boardID=122&ID=146580
MySQL权威指南 中文版+英文版
http://bbs.topsage.com/dispbbs.asp?boardID=122&ID=148870
MySQL 4.1.0 中文参考手册
http://bbs.topsage.com/dispbbs.asp?boardID=122&ID=148869
MySQL in a Nutshell
http://bbs.topsage.com/dispbbs.asp?boardID=122&ID=148868
Export MySQL
http://bbs.topsage.com/dispbbs.asp?boardID=122&ID=152865
MySQL and php from Scratch
http://bbs.topsage.com/dispbbs.asp?boardID=122&ID=152162 参考技术B 当然可以，数据库中有系统表记录这些约束 的，不知你用的是什么数据库，你可以自己找找 参考技术C SQL-server数据库中，约束都保存在你当前库的系统表sysobjects表中，例如要添加约束：
if exists(select * from sysobjects where name=约束名)
drop constraint 约束名
alter table 表名
add constraint 约束本回答被提问者采纳 参考技术D 愿意的话等我半年。如今我刚学VFP，之后考级。现在我无能为力哦。