How does SQL modify constraints for added columns?

SQL cannot directly modify the constraints of added columns, but needs to save the country through the following methods: Delete reconstruction: Rebuild after deleting the original constraint, but the risk is huge. When the data is large, it will take time and lead to temporary unavailability; New column migration: Create new columns and copy data, and then delete old columns, which is relatively safe but troublesome; use views: Create views to apply new constraints, and will not modify the underlying table, but only take effect at the view level.

How does SQL modify constraints for added columns? This question seems simple, but in fact it has hidden mystery. If you are not careful, you will fall into the pit. Many beginners think that adding CONSTRAINT to ALTER TABLE is done, but in actual operation, you will find that things are far from that easy.

Let’s talk about the conclusion first: you cannot directly modify the constraints of added columns. The constraint mechanism of SQL, especially the constraint modification part of the ALTER TABLE command, is designed to restrict direct modification of existing column constraints. This is not a "deliberate difficulty" by database manufacturers, but is due to the consideration of data consistency and transaction integrity. Imagine how much time the database will take to check if the data still complies with the new constraints if it is allowed to modify the existing constraints directly? This can seriously affect the performance of the database.

So, what should I do? Only save the country in a curve. There are several methods. I choose a few commonly used ones and talk about their respective advantages and disadvantages, as well as the pitfalls I have stepped on.

Method 1: Delete and rebuild

This is the most direct and violent method. First use ALTER TABLE to delete the original constraint, and then add a new constraint. The code example is as follows (assuming you want to modify the constraint named my_column column in the table named my_table ):

 <code class="sql">ALTER TABLE my_table DROP CONSTRAINT my_constraint; -- 删除原约束，my_constraint替换成你的约束名ALTER TABLE my_table ADD CONSTRAINT my_constraint CHECK (my_column > 0); -- 添加新约束</code>

Advantages: simple and crude, easy to understand.

Disadvantages: The risk is huge! If the table data is huge, this process can be very time-consuming and even lead to the database being temporarily unavailable. More importantly, the database will be inconsistent during deletion constraints, which is a disaster in a high concurrency environment. I used to temporarily paralyze online services because of this method, and that feeling... is hard to describe. Therefore, this method is strongly not recommended unless the table data volume is extremely small.

Method 2: Use new columns and data migration

Create a new column with the constraints you want. Then, copy the data from the old column to the new column. Finally, delete the old column. This method is more complicated, but safer.

 <code class="sql">ALTER TABLE my_table ADD COLUMN my_column_new INT CHECK (my_column_new > 0); UPDATE my_table SET my_column_new = my_column; ALTER TABLE my_table DROP COLUMN my_column; ALTER TABLE my_table RENAME COLUMN my_column_new TO my_column;</code>

Advantages: Safe and reliable, avoiding the risk of data inconsistency.

Disadvantages: It requires additional steps, which is more troublesome. Moreover, if your table has foreign key associations, the process will become more complicated.

Method 3: Utilizing views

You can create a view in which new constraints are applied. This does not modify the underlying table structure, but allows you to apply new constraints when querying. This is a tradeoff.

 <code class="sql">CREATE VIEW my_view AS SELECT * FROM my_table WHERE my_column > 0;</code>

Advantages: It is safe and reliable without modifying the original table structure.

Disadvantages: Just apply constraints at the view level, the underlying table data may still not comply with the new constraints. This is not applicable to scenarios where data integrity requirements are very high.

Which method to choose depends on your specific situation. Data volume, concurrency volume, and data integrity requirements will affect your choice. Remember, safety is always the first priority. When modifying the database structure in a production environment, you must be cautious and cautious. It is best to conduct sufficient testing in the test environment first. Don't forget to back up your data! This is the way to survive only veteran drivers understand.

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