在现有 MySQL 表中添加具有唯一键索引的新字段

Question

I am trying to add a column with Unique Key (so it will not have duplicate records) in existing MySQL table that contains multiple rows of data.

ALTER TABLE  `common`.`fraud_payment_log`
  ADD  `retainer_id` VARCHAR( 20 ) NOT NULL,
  ADD  `http_referrer` VARCHAR( 255 ) NULL ,
  ADD UNIQUE (`retainer_id`);

But it is throwing below error:

ERROR 1062 (23000): Duplicate entry '' for key 'retainer_id'

The error is because of the duplicate empty value which will come when we adding a new column in the existing table with records.

Can anyone please suggest how to achieve this?

Answer 1

You cannot add a unique, non-NULL column to a table that has more than one row. By definition, two rows would get the same value.

Add the columns first, allowing NULL values:

ALTER TABLE  `common`.`fraud_payment_log`
    ADD  `retainer_id` VARCHAR( 20 ) NULL,
    ADD  `http_referrer` VARCHAR( 255 ) NULL;

Now, populate the column so it has different values. Say:

update `common`.`fraud_payment_log` cross join
       (select @rn := 0) vars
     set retainer_id = (@rn := @rn + 1);

Then add the unique constraint. I usually do this with the index directly:

create unique index idx_fpl_retainer on  `common`.`fraud_payment_log`(retainer_id);

If the table is empty, then just recreate the table with all the columns you want.

Answer 2

Follow this steps:

• Add new field which is not unique
• Update table and put unique values in that field
• Alter table to make this field as unique

Answer 3

您应该首先添加列，然后用唯一值填充它，然后添加UNIQUE约束。

Answer 4

You should specify a DEFAULT NULL ; NULL doesn't count against UNIQUE checks, so your table will be first filled with NULL values, then you'll proceed to fill it as you wish.

Answer 5

1. Create new column using UI or query(My new column is sensorID )
2. Fire this :

UPDATE client set sensorID=id

3. In above query client is my table , sensorID is my newly added column on which I wanted to apply unique, id is my primary key type column in existing table.