无法在 Big Query 中更改列数据类型

Question

We imported database into BigQuery but a lot of columns are not in correct data types, many of them are stored as STRING. I want to fix them by change the data type in BigQuery

ALTER TABLE my.data_set.my_table ALTER COLUMN create_date SET DATA TYPE DATE;

But I got

ALTER TABLE ALTER COLUMN SET DATA TYPE requires that the existing column type (STRING) is assignable to the new type (DATE)

How to solve it?

Answer 1

Unfortunately, as far as I know there is no way to convert data type from STRING to DATE using ALTER TABLE but to create it again with the schema you want.

CREATE OR REPLACE TABLE testset.tbl AS 
SELECT 'a' AS col1, '2022-05-16' AS create_date
 UNION ALL
SELECT 'a' AS col1, '2022-05-14' AS create_date
;

-- ALTER TABLE testset.tbl ALTER COLUMN create_date SET DATA TYPE DATE;

-- Query error: ALTER TABLE ALTER COLUMN SET DATA TYPE requires that
-- the existing column type (STRING) is assignable to the new type (DATE) at [7:25]


-- Create it again. 
CREATE OR REPLACE TABLE testset.tbl AS
SELECT * REPLACE(SAFE_CAST(create_date AS DATE) AS create_date) 
  FROM testset.tbl;

Answer 2

Follow the steps below:

1. Rename the existing column

ALTER TABLE mydataset.mytable RENAME COLUMN create_date TO create_date_to_drop;

2. Add the column with the required data type.

 ALTER TABLE mydataset.mytable ADD COLUMN create_date DATE;

3. Drop the renamed column.

 ALTER TABLE mydataset.mytable DROP COLUMN create_date_to_drop;

Note: As of now, DROP COLUMN won't work on the tables protected by customer-managed encryption keys.

Alternatively, you can drop the existing column in the beta version and add it with the new data type, but you need to wait until the time travel window (7 days by default) to get over for adding the column with the same name.