i have created a table with 3 columns in which 2 are defined as NOT NULL in SQL, now i created a new column i wanted to insert values to only the new variable, but i'm having an error while using Insert into statement
If you are using MySQL, here is your answer:
Inserting NULL into a column that has been declared NOT NULL. For multiple-row INSERT statements or INSERT INTO ... SELECT statements, the column is set to the implicit default value for the column data type. This is 0 for numeric types, the empty string ('') for string types, and the “zero” value for date and time types. INSERT INTO ... SELECT statements are handled the same way as multiple-row inserts because the server does not examine the result set from the SELECT to see whether it returns a single row. (For a single-row INSERT, no warning occurs when NULL is inserted into a NOT NULL column. Instead, the statement fails with an error.)
The documentation is here: MySQL 5.7 Reference Manual: Insert
As said above, just use ' ' and 0 depending on the type of column you have. I believe it's the same for any other DBMS system anyone could use.
Disregarding that this normally indicates you should rethink your database design, some database engines will let you do this through temporary schema manipulation (removing and afterwards re-adding the 'not null' constraint).
Alternatively you could either define default values for the not null columns, or pass in through the insert command the appropriate "default" values, or select a database mode where either the checks are not enforced (thus allowing nulls) or default values are automatically generated (eg mysql in non strict mode which replaces the nulls with a calculated default value).
The only valid use case I can think of is replicating the situation where a database has rows with nulls in certain fields, and then the schema is changed to make those columns not null. Mysql for instance will allow you to add 'not null' constraints to columns that already have data with nulls in that column.
Related
I'm trying to create a view where I want a column to be only true or false. However, it seems that no matter what I do, SQL Server (2008) believes my bit column can somehow be null.
I have a table called "Product" with the column "Status" which is INT, NULL. In a view, I want to return a row for each row in Product, with a BIT column set to true if the Product.Status column is equal to 3, otherwise the bit field should be false.
Example SQL
SELECT CAST( CASE ISNULL(Status, 0)
WHEN 3 THEN 1
ELSE 0
END AS bit) AS HasStatus
FROM dbo.Product
If I save this query as a view and look at the columns in Object Explorer, the column HasStatus is set to BIT, NULL. But it should never be NULL. Is there some magic SQL trick I can use to force this column to be NOT NULL.
Notice that, if I remove the CAST() around the CASE, the column is correctly set as NOT NULL, but then the column's type is set to INT, which is not what I want. I want it to be BIT. :-)
You can achieve what you want by re-arranging your query a bit. The trick is that the ISNULL has to be on the outside before SQL Server will understand that the resulting value can never be NULL.
SELECT ISNULL(CAST(
CASE Status
WHEN 3 THEN 1
ELSE 0
END AS bit), 0) AS HasStatus
FROM dbo.Product
One reason I actually find this useful is when using an ORM and you do not want the resulting value mapped to a nullable type. It can make things easier all around if your application sees the value as never possibly being null. Then you don't have to write code to handle null exceptions, etc.
FYI, for people running into this message, adding the ISNULL() around the outside of the cast/convert can mess up the optimizer on your view.
We had 2 tables using the same value as an index key but with types of different numerical precision (bad, I know) and our view was joining on them to produce the final result. But our middleware code was looking for a specific data type, and the view had a CONVERT() around the column returned
I noticed, as the OP did, that the column descriptors of the view result defined it as nullable and I was thinking It's a primary/foreign key on 2 tables; why would we want the result defined as nullable?
I found this post, threw ISNULL() around the column and voila - not nullable anymore.
Problem was the performance of the view went straight down the toilet when a query filtered on that column.
For some reason, an explicit CONVERT() on the view's result column didn't screw up the optimizer (it was going to have to do that anyway because of the different precisions) but adding a redundant ISNULL() wrapper did, in a big way.
All you can do in a Select statement is control the data that the database engine sends to you as a client. The select statement has no effect on the structure of the underlying table. To modify the table structure you need to execute an Alter Table statement.
First make sure that there are currently no nulls in that bit field in the table
Then execute the following ddl statement:
Alter Table dbo.Product Alter column status bit not null
If, otoh, all you are trying to do is control the output of the view, then what you are doing is sufficient. Your syntax will guarantee that the output of the HasStatus column in the views resultset will in fact never be null. It will always be either bit value = 1 or bit value = 0. Don't worry what the object explorer says...
Update:
Here is what I have learned from the answers below...
SQLite supports defining default values on columns. However, it does not support any form of DEFAULT keyword or function in queries. So, in effect, it is not possible to do a MySQL style bulk insert where a column is set explicitly in some rows, and the column default is used in other rows.
There are several creative workarounds listed below, but for simplicity's sake I most prefer Shawn's suggestion of using an INSERT statement per row, possibly all wrapped inside a BEGIN TRANSACTION statement. Only this will allow you to utilize column defaults.
Original Question:
I have a simple SQLite table named todo with this schema...
I can insert rows into this table with this syntax...
INSERT INTO 'todo' ('title', 'complete', 'notes') VALUES
('Pickup dry cleaning', true, 'They close at 7pm'),
('Clean apartment', false, 'Floors have already been swept'),
('Finish homework', true, 'Chapters 3 and 4');
However,
I can't figure out how to omit certain values and let the database use the default for that column.
For example, the complete column has a default of 0. It would be nice to be able to pass undefined as one of the values in my query, thus signaling to SQLite that I'd like to use whatever the column default is, but I get a Result: no such column: undefined error when I try this.
null would not work in this case either, because sometimes you want to explicitly set the value of a cell to null. You can't just plain omit the value either because the parameters are positional.
You just leave it out of the columns being inserted:
INSERT INTO todo(title, notes) VALUES ('Music practice', 'Not Elvis');
I can't figure out how to omit certain values and let the database use the default for that column.
As of this writing, SQLite does not support the default(_) function, but you could use triggers to achieve the effect you want, though doing so is a bit cumbersome.
Here's an example:
CREATE TRIGGER IF NOT EXISTS onnull
AFTER INSERT ON mytable
FOR EACH ROW
WHEN NEW.column = 'undefined'
BEGIN
UPDATE mytable
SET column = 'mydefault' ;
END;
this works if you are using better-sqlite3, and I think it will equally work for sqlite3.
db.prepare("INSERT INTO bugs (title, notes) VALUES (?, ?)").run(["Good Book", "nice test"]);
What Shawn said plus the bulk issue can be solved with DEFAULT value, that is if you are inserting all records manually, if you want it dynamically through a loop for example you must declare a variable as default and change to the specific value if the specific value exists, and use this variable in the query.
String var='DEFAULT';
if (Value exists) {var=Value;}
sql=inset into x(x,y,z) values (a,b,var);
How do I alter a sql varchar column to a decimal column when there are nulls in the data?
I thought:
ALTER TABLE table1
ALTER COLUMN data decimal(19,6)
But I just get an error, I assume because of the nulls:
Error converting data type varchar to numeric. The statement has been terminated.
So I thought to remove the nulls I could just set them to zero:
ALTER TABLE table1
ALTER COLUMN data decimal(19,6) NOT NULL DEFAULT 0
but I dont seem to have the correct syntax.
Whats the best way to convert this column?
edit
People have suggested it's not the nulls that are causing me the problem, but non-numeric data. Is there an easy way to find the non-numeric data and either disregard it, or highlight it so I can correct it.
If it were just the presence of NULLs, I would just opt for doing this before the alter column:
update table1 set data = '0' where data is null
That would ensure all nulls are gone and you could successfully convert.
However, I wouldn't be too certain of your assumption. It seems to me that your new column is perfectly capable of handling NULL values since you haven't specified not null for it.
What I'd be looking for is values that aren't NULL but also aren't something you could turn in to a real numeric value, such as what you get if you do:
insert into table1 (data) values ('paxdiablo is good-looking')
though some may argue that should be treated a 0, a false-y value :-)
The presence of non-NULL, non-numeric data seems far more likely to be causing your specific issue here.
As to how to solve that, you're going to need a where clause that can recognise whether a varchar column is a valid numeric value and, if not, change it to '0' or NULL, depending on your needs.
I'm not sure if SQL Server has regex support but, if so, that'd be the first avenue I'd investigate.
Alternatively, provided you understand the limitations (a), you could use isnumeric() with something like:
update table1 set data = NULL where isnumeric(data) = 0
This will force all non-numeric values to NULL before you try to convert the column type.
And, please, for the love of whatever deities you believe in, back up your data before attempting any of these operations.
If none of those above solutions work, it may be worth adding a brand new column and populating bit by bit. In other words set it to NULL to start with, and then find a series of updates that will copy data to this new column.
Once you're happy that all data has been copied, you should then have a series of updates you can run in a single transaction if you want to do the conversion in one fell swoop. Drop the new column and then do the whole lot in a single operation:
create new column;
perform all updates to copy data;
drop old column;
rename new column to old name.
(a) From the linked page:
ISNUMERIC returns 1 for some characters that are not numbers, such as plus (+), minus (-), and valid currency symbols such as the dollar sign ($).
Possible solution:
CREATE TABLE test
(
data VARCHAR(100)
)
GO
INSERT INTO test VALUES ('19.01');
INSERT INTO test VALUES ('23.41');
ALTER TABLE test ADD data_new decimal(19,6)
GO
UPDATE test SET data_new = CAST(data AS decimal(19,6));
ALTER TABLE test DROP COLUMN data
GO
EXEC sp_RENAME 'test.data_new' , 'data', 'COLUMN'
As people have said, that error doesn't come from nulls, it comes from varchar values that can't be converted to decimal. Most typical reason for this I've found (after checking that the column doesn't contain any logically false values, like non-digit characters or double comma values) is when your varchar values use comma for decimal pointer, as opposed to period.
For instance, if you run the following:
DECLARE #T VARCHAR(256)
SET #T = '5,6'
SELECT #T, CAST(#T AS DEC(32,2))
You will get an error.
Instead:
DECLARE #T VARCHAR(256)
SET #T = '5,6'
-- Let's change the comma to a period
SELECT #T = REPLACE(#T,',','.')
SELECT #T, CAST(#T AS DEC(32,2)) -- Now it works!
Should be easy enough to look if your column has these cases, and run the appropriate update before your ALTER COLUMN, if this is the cause.
You could also just use a similar idea and make a regex search on the column for all values that don't match digit / digit+'.'+digit criteria, but i suck with regex so someone else can help with that. :)
Also, the american system uses weird separators like the number '123100.5', which would appear as '123,100.5', so in those cases you might want to just replace the commas with empty strings and try then?
select field from table where field = 'value'
select field from table where field in ('value')
The reason I'm asking is that the second version allow me to use the same syntax for null values, while in the first version I need to change the condition to 'where field is null'...
When you are comparing a field to a null like field_name=NULL you are comparing to a known data type from a field say varchar to not only an unknown value but also an unknown data type as well, that is, for NULL values. When comparison like field_name=NULL again implies therefore a checking of data type for both and thus the two could not be compared even if the value of the field is actually NULL thus it will always result to false. However, using the IS NULL you are only comparing for the value itself without the implied comparison for data type thus it could result either to false or true depending on the actual value of the field.
See reference here regarding the issue of NULL in computer science and here in relation to the similarity to your question.
Now, for the IN clause (i.e. IN(NULL)) I don't know what RDBMS you are using because when I tried it with MS SQL and MySQL it results to nothing.
See MS SQL example and MySQL example.
There is no difference in your example. The second, slightly longer, query is not usually used for a single value, it is usally seen for multiple values, such as
select field from table where field in ('value1', 'value2')
yes there is difference in both this queries. In first statment you can insert only 1 value in where clause "where field = 'value'" but in second statement in where field you can insert many values using IN clause "where field in (value1,value2..)"
Examples:
1) select field from table where field ='value1';
2) select field from table where field in ('value1', 'value2')
To check null values
SELECT field
FROM tbl_name
WHERE
(field IN ('value1', 'value2', 'value3') OR field IS NULL)
I have a table that looks a bit like this actors(forename, surname, stage_name);
I want to update stage_name to have a default value of
forename." ".surname
So that
insert into actors(forename, surname) values ('Stack', 'Overflow');
would produce the record
'Stack' 'Overflow' 'Stack Overflow'
Is this possible?
Thanks :)
MySQL does not support computed columns or expressions in the DEFAULT option of a column definition.
You can do this in a trigger (MySQL 5.0 or greater required):
CREATE TRIGGER format_stage_name
BEFORE INSERT ON actors
FOR EACH ROW
BEGIN
SET NEW.stage_name = CONCAT(NEW.forename, ' ', NEW.surname);
END
You may also want to create a similar trigger BEFORE UPDATE.
Watch out for NULL in forename and surname, because concat of a NULL with any other string produces a NULL. Use COALESCE() on each column or on the concatenated string as appropriate.
edit: The following example sets stage_name only if it's NULL. Otherwise you can specify the stage_name in your INSERT statement, and it'll be preserved.
CREATE TRIGGER format_stage_name
BEFORE INSERT ON actors
FOR EACH ROW
BEGIN
IF (NEW.stage_name IS NULL) THEN
SET NEW.stage_name = CONCAT(NEW.forename, ' ', NEW.surname);
END IF;
END
According to 10.1.4. Data Type Default Values no, you can't do that. You can only use a constant or CURRENT_TIMESTAMP.
OTOH if you're pretty up-to-date, you could probably use a trigger to accomplish the same thing.
My first thought is if you have the two values in other fields what is the compelling need for redundantly storing them in a third field? It flies in the face of normalization and efficiency.
If you simply want to store the concatenated value then you can simply create a view (or IMSNHO even better a stored procedure) that concatenates the values into a pseudo actor field and perform your reads from the view/sproc instead of the table directly.
If you absolutely must store the concatenated value you could handle this in two ways:
1) Use a stored procedure to do your inserts instead of straight SQL. This way you can receive the values and construct a value for the field you wish to populate then build the insert statement including a concatenated value for the actors field.
2) So I don't draw too many flames, treat this suggestion with kid gloves. Use only as a last resort. You could hack this behavior by adding a trigger to build the value if it is left null. Generally, triggers are not good. They add unseen cost and interactions to fairly simple interactions. You can, though, use the CREATE TRIGGER to update the actors field after a record is inserted or updated. Here is the reference page.
As of MySQL 8.0.13, you can use DEFAULT clause for a column which can be a literal constant or an expression.
If you want to use an expression then, simply enclose the required expression within parentheses.
(concat(forename," ",surname))
There are two ways to accomplish what you are trying to do as per my knowledge:
(important: consider backing up your table first before running below queries)
1- Drop the column "stage_name" all together and create a new one with DEFAULT constraint.
ALTER TABLE actors ADD COLUMN stage_name VARCHAR(20) DEFAULT (concat(forename," ",surname))
2- This will update newer entries in the column "stage_name" but not the old ones.
ALTER TABLE actors alter stage_name set DEFAULT (concat(forename," ",surname));
After that, if you need to update the previous values in the column "stage_name" then simply run:
UPDATE actors SET stage_name=(concat(forename," ",surname));
I believe this should solve your problem.