Does CockroachDB support default values for columns in its tables? Does it allow default values to be function values (e.g. current_date())?
You can set DEFAULT values using the DEFAULT constraint, which CockroachDB has documented here.
It also supports setting the default value as a function, e.g. to insert the date that a write occurred.
You would create a table with such a default column like:
CREATE TABLE purchase_log (
id INT PRIMARY KEY,
date_purchased DATE DEFAULT current_date()
);
Then all inserts to the table that don't specify the date_purchased column will have the column automatically populated with the return value of current_date() at the time of the insert.
Related
From the documentation, you have to put a NOT NULL modifier in the column definition to mark it as such, just like for other SQL databases.
Consider this table:
CREATE TABLE test (
name String NOT NULL,
isodate DateTime('Europe/Berlin') NOT NULL
) ENGINE = MergeTree()
ORDER BY (isodate)
If I would try to insert NULL for both columns (or at least one), the expected behaviour is that Clickhouse declines insertion since the columns are marked as NOT NULL. Instead, Clickhouse creates a new row, where isodate is 1970-01-01 01:00:00 and name is an empty string, which are the default values for those data types apparently.
What do I have to do so that Clickhouse declines such inserts?
My Clickhouse server version is 21.12.3.
In ClickHouse, NULL and NOT NULL do change the behavior of the data type, but not in the way other relational databases - it is syntactically compatible with other relational database but not semantically (a Int32 NULL is the same as a Nullable(Int32), as a Int32 NOT NULL is the same as a Int32). A column defined as NOT NULL does not mean it will refuse to insert fields whose values are NULL in the insert statement - it means ClickHouse will use the default expression for the column type (or if it is not specified in the column definition, the default value for the data type). This behavior is expected in ClickHouse when input_format_null_as_default is enabled (the default for Clickhouse 21.12.3).
To throw exceptions for such invalid values you need to change the system setting input_format_null_as_default to 0. If you use clickhouse-client, you can disable it while connecting to clickhouse:
clickhouse-client -h ... --input_format_null_as_default 0
or after:
clickhouse> SET input_format_null_as_default=0
This way, a statement like insert into test (name, isodate) values (NULL, NULL); will behave more likely most relational databases.
Clickhouse behaviour with Not Null constraints is not compatible with other databases.
You can overcome it using check constraints https://clickhouse.com/docs/en/sql-reference/statements/create/table/#constraints
CREATE TABLE test (
name String NOT NULL,
isodate DateTime('Europe/Berlin') NOT NULL,
CONSTRAINT isodate_not_null CHECK isodate <> toDateTime(0, 'Europe/Berlin')
) ENGINE = MergeTree()
ORDER BY (isodate)
insert into test(name) values ('x');
DB::Exception: Constraint `isodate_not_null` for table default.test (f589312a-1592-426a-b589-312a1592b26a) is violated at row 1. Expression: (isodate != toDateTime(0)). Column values: isodate = 0. (VIOLATED_CONSTRAINT)
insert into test values ('x', now());
OK.
The reason is performance, in OLAP databases need to ingest data as fast as possible.
We have a very large table (1 Million records) in some cases and we need to add boolean fields to it which have default values.
If we add only column it takes 3 minutes and we add 3 columns in the same statement it takes same time.
$ALTER TABLE Job ADD COLUMN test BOOLEAN NOT NULL default false;
ALTER TABLE
Time: 186506.603 ms
$ALTER TABLE Job ADD COLUMN test BOOLEAN NOT NULL default false ,
ADD COLUMN test1 BOOLEAN NOT NULL default false,
ADD COLUMN test2 BOOLEAN NOT NULL default false;
ALTER TABLE
Time: 179055.546 ms
We are on Postgres 9.1 . Is there a postgres feature allows to add multiple boolean fields with default values in one shot? This is for a database change management /upgrade solution . Is it better than using temp table to copy and insert to add multiple boolean fields with default values to a table ? The temp table approach is described in this blog:
http://blog.codacy.com/2015/05/14/how-to-update-large-tables-in-postgresql/
You've already shown the best (simple) way - a compound ALTER TABLE statement that adds them all at once.
To do it without a long lock, you have to do it in multiple steps. Add the column as nullable without the default. Add the default but leave it nullable. UPDATE all existing rows to add the new value, preferably in batches. Then finally alter the table to add the not null constraint.
I created a table named- books and have a column in that by the title 'color' . Initially I have null values in the column 'color'. Now, when I run the following query :
alter table books modify color default 'blue';
schema is formed but on doing select *from books , all the values in column color are still null. What would be the correct query to fire?
here is the link:
http://sqlfiddle.com/#!4/f4210/1
Of course. Alter table just changes the table structure but not the content. New entries will get the default.
To update the existing values run a sql-update query like:
update books set color='blue' where colore is null;
If you now inserting into table then only will come with default values. This statement don't know about previous contents of this table. In non technical language, you are telling oracle to do so now on-wards. This statement will not perform check to old values.
alter is ok for the next values to be inserted: try to insert lines without specifying a value for column color, value should be blue.
But this does not work for existing values, for which you just need an update:
update books set color = 'blue';
Hi this query will be used to add column with default value in existing table in oracle.
alter table <table_name> add <column_name> <contraint> default <default_value> not null;
example:
alter table books add record_status number(1,0) default 1 not null;
alter table books add color varchar(20) default 'blue' not null;
I have a column with REAL data type, and I'm trying to write a statement to change the default value to 4. However, when I use the select * statement to double check, the column is still empty.
ALTER TABLE recipes
MODIFY NumberOfServings DEFAULT '4';
The DEFAULT value is used at INSERT time. It gives a default value to be inserted when you do not provide a value for the column in the INSERT statement.
You may want to update the table, so that all NULL values are replaced by a given value:
UPDATE recipes SET NumberOfServings=4 WHERE NumberOfServings IS NULL;
You can also specify a value to use when the column is NULL at QUERY time.
This can be done by using the NVL function:
SELECT NVL(NumberOfServings,4) FROM recipes;
http://docs.oracle.com/cd/B19306_01/server.102/b14200/functions105.htm
The default value applies only when you insert new records into the table. It does not update any existing values in the table. Why do you enclose a numeric value in quotes?
why I am getting this error ?
In the table DDL I only have 2 columns , id (number) and name (varchar)
ALTER TABLE mytable ADD SUSPEND date NOT NULL
ORA-01758: table must be empty to add mandatory (NOT NULL) column
ORA-06512: at line 7
ORA-01758: table must be empty to add mandatory (NOT NULL) column ORA-06512: at line 7
And is your table empty? I think not.
There's probably a way around this involving adding the column as nullable, then populating every row with a non-NULL value, the altering the column to be not null.
Alternatively, since the problem is that these current rows will be given NULL as a default value, and the column is not allowed to be NULL, you can also get around it with a default value. From the Oracle docs:
However, a column with a NOT NULL constraint can be added to an existing table if you give a default value; otherwise, an exception is thrown when the ALTER TABLE statement is executed.
Here is a fiddle, how you could do it
Would a date in the future be acceptable as a temporary default? If so, this would work:
ALTER TABLE MYTABLE ADD (SUSPEND_DATE DATE DEFAULT(TO_DATE('21000101', 'YYYYMMDD'))
CONSTRAINT SUSPEND_DATE_NOT_NULL NOT NULL);
If table already contain the records then table won't allowes to add "Not null" column.
If you need same then set default value for the column or truncate the table then try.