I have a column where it stores a reference
the reference is usually 30 alphanumeric values
The column is set as an nvarchar (30).
Id like to update this to nvarchar (5), and update all the values currently stored in this column..
is there a way to do this using update?
First update the table so that you truncate all the column values after the 5th character:
UPDATE tablename SET columnname = LEFT(columnname, 5);
Note, that this may not work if there are constraints on the column (like a unique constraint or foreign key references).
Then change the data type of the column:
ALTER TABLE tablename ALTER COLUMN columnname nvarchar(5);
See a simplified demo.
Should do the trick on SQL Server / MS Access:
ALTER TABLE table_name
ALTER COLUMN column_name datatype;
How do I remove AUTO_INCREMENT of a column in DB2?
I tried
ALTER TABLE my_table ALTER my_id
but no luck. What is the correct SQL statement?
You can modify a column definition of a DB2 table and remove the generation of values:
ALTER TABLE mytable ALTER COLUMN mycol DROP GENERATED;
You can remove the Auto_increment by using the following
ALTER TABLE table_name ALTER COLUMN column_name DROP IDENTITY;
I would like to write a single SQL command to drop multiple columns from a single table in one ALTER TABLE statement.
From MSDN's ALTER TABLE documentation...
DROP { [CONSTRAINT] constraint_name | COLUMN column_name }
Specifies that constraint_name or column_name is removed from the table. DROP COLUMN is not allowed if the compatibility level is 65 or earlier. Multiple columns and constraints can be listed.
It says that mutliple columns can be listed in the the statement but the syntax doesn't show an optional comma or anything that would even hint at the syntax.
How should I write my SQL to drop multiple columns in one statement (if possible)?
For SQL Server:
ALTER TABLE TableName
DROP COLUMN Column1, Column2;
The syntax is
DROP { [ CONSTRAINT ] constraint_name | COLUMN column } [ ,...n ]
For MySQL:
ALTER TABLE TableName
DROP COLUMN Column1,
DROP COLUMN Column2;
or like this1:
ALTER TABLE TableName
DROP Column1,
DROP Column2;
1 The word COLUMN is optional and can be omitted, except for RENAME COLUMN (to distinguish a column-renaming operation from the RENAME table-renaming operation). More info here.
Summarizing
Oracle:
ALTER TABLE table_name DROP (column_name1, column_name2);
MS SQL Server:
ALTER TABLE table_name DROP COLUMN column_name1, column_name2
MySQL:
ALTER TABLE table_name DROP column_name1, DROP column_name2;
PostgreSQL
ALTER TABLE table_name DROP COLUMN column_name1, DROP COLUMN column_name2;
Be aware
DROP COLUMN does not physically remove the data for some DBMS. E.g. for MS SQL. For fixed length types (int, numeric, float, datetime, uniqueidentifier etc) the space is consumed even for records added after the columns were dropped. To get rid of the wasted space do ALTER TABLE ... REBUILD.
create table test (a int, b int , c int, d int);
alter table test drop column b, d;
Be aware that DROP COLUMN does not physically remove the data, and for fixed length types (int, numeric, float, datetime, uniqueidentifier etc) the space is consumed even for records added after the columns were dropped. To get rid of the wasted space do ALTER TABLE ... REBUILD.
This may be late, but sharing it for the new users visiting this question.
To drop multiple columns actual syntax is
alter table tablename drop column col1, drop column col2 , drop column col3 ....
So for every column you need to specify "drop column" in Mysql 5.0.45.
The Syntax as specified by Microsoft for the dropping a column part of an ALTER statement is this
DROP
{
[ CONSTRAINT ]
{
constraint_name
[ WITH
( <drop_clustered_constraint_option> [ ,...n ] )
]
} [ ,...n ]
| COLUMN
{
column_name
} [ ,...n ]
} [ ,...n ]
Notice that the [,...n] appears after both the column name and at the end of the whole drop clause. What this means is that there are two ways to delete multiple columns. You can either do this:
ALTER TABLE TableName
DROP COLUMN Column1, Column2, Column3
or this
ALTER TABLE TableName
DROP
COLUMN Column1,
COLUMN Column2,
COLUMN Column3
This second syntax is useful if you want to combine the drop of a column with dropping a constraint:
ALTER TBALE TableName
DROP
CONSTRAINT DF_TableName_Column1,
COLUMN Column1;
When dropping columns SQL Sever does not reclaim the space taken up by the columns dropped. For data types that are stored inline in the rows (int for example) it may even take up space on the new rows added after the alter statement. To get around this you need to create a clustered index on the table or rebuild the clustered index if it already has one. Rebuilding the index can be done with a REBUILD command after modifying the table. But be warned this can be slow on very big tables. For example:
ALTER TABLE Test
REBUILD;
For MySQL (ver 5.6), you cannot do multiple column drop with one single drop-statement but rather multiple drop-statements:
mysql> alter table test2 drop column (c1,c2,c3);
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '(c1,c2,c3)' at line 1
mysql> alter table test2 drop column c1,c2,c3;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'c2,c3' at line 1
mysql> alter table test2 drop column c1, drop column c2, drop c3;
Query OK, 0 rows affected (0.64 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql>
BTW, drop <col_name> is shorthanded for drop column <col_name> as you can see from drop c3 above.
If it is just single column to delete the below syntax works
ALTER TABLE tablename DROP COLUMN column1;
For deleting multiple columns, using the DROP COLUMN doesnot work, the below syntax works
ALTER TABLE tablename DROP (column1, column2, column3......);
Generic:
ALTER TABLE table_name
DROP COLUMN column1,column2,column3;
E.g:
ALTER TABLE Student
DROP COLUMN Name, Number, City;
alter table tablename drop (column1, column2, column3......);
for postgis is
alter table table01 drop columns col1, drop col2
this query will alter the multiple column test it.
create table test(a int,B int,C int);
alter table test drop(a,B);
ALTER table table_name Drop column column1, Drop column column2,Drop column column3;
for MySQL DB.
Or you can add some column while altering in the same line:
ALTER table table_name Drop column column1, ADD column column2 AFTER column7;
I want to alter a table column to be nullable. I have used:
ALTER TABLE Merchant_Pending_Functions Modify NumberOfLocations NULL
This gives an error at Modify. What is the correct syntax?
Assuming SQL Server (based on your previous questions):
ALTER TABLE Merchant_Pending_Functions ALTER COLUMN NumberOfLocations INT NULL
Replace INT with your actual datatype.
If this was MySQL syntax, the type would have been missing, as some other responses point out.
Correct MySQL syntax would have been:
ALTER TABLE Merchant_Pending_Functions MODIFY NumberOfLocations INT NULL
Posting here for clarity to MySQL users.
In PostgresQL it is:
ALTER TABLE tableName ALTER COLUMN columnName DROP NOT NULL;
for Oracle Database 10g users:
alter table mytable modify(mycolumn null);
You get "ORA-01735: invalid ALTER TABLE option" when you try otherwise
ALTER TABLE mytable ALTER COLUMN mycolumn DROP NOT NULL;
Although I don't know what RDBMS you are using, you probably need to give the whole column specification, not just say that you now want it to be nullable. For example, if it's currently INT NOT NULL, you should issue ALTER TABLE Merchant_Pending_Functions Modify NumberOfLocations INT.
As others have observed, the precise syntax for the command varies across different flavours of DBMS. The syntax you use works in Oracle:
SQL> desc MACAddresses
Name Null? Type
----------------------------------------- -------- ----------------------------
COMPUTER NUMBER
MACADDRESS VARCHAR2(12)
CORRECTED_MACADDRESS NOT NULL VARCHAR2(17)
SQL> alter table MACAddresses
2 modify corrected_MACAddress null
3 /
Table altered.
SQL> desc MACAddresses
Name Null? Type
----------------------------------------- -------- ----------------------------
COMPUTER NUMBER
MACADDRESS VARCHAR2(12)
CORRECTED_MACADDRESS VARCHAR2(17)
SQL>
For SQL Server or TSQL
ALTER TABLE Complaint.HelplineReturn ALTER COLUMN IsDisposed BIT NULL
This depends on what SQL Engine you are using, in Sybase your command works fine:
ALTER TABLE Merchant_Pending_Functions
Modify NumberOfLocations NULL;
For HSQLDB:
ALTER TABLE tableName ALTER COLUMN columnName SET NULL;
ALTER TABLE Merchant_Pending_Functions MODIFY COLUMN `NumberOfLocations` INT null;
This will work for you.
If you want to change a not null column to allow null, no need to include not null clause. Because default columns get not null.
ALTER TABLE Merchant_Pending_Functions MODIFY COLUMN `NumberOfLocations` INT;
Oracle
ALTER TABLE Merchant_Pending_Functions MODIFY([column] NOT NULL);
SQLite
The ALTER TABLE command is a bit special. There is no possibility to modify a column. You have to create a new column, migrate the data, and then drop the column:
-- 1. First rename
ALTER TABLE
Merchant_Pending_Functions
RENAME COLUMN
NumberOfLocations
TO
NumberOfLocations_old
-- 2. Create new column
ALTER TABLE
Merchant_Pending_Functions
ADD COLUMN
NumberOfLocations INT NULL
-- 3. Migrate data - you need to write code for that
-- 4. Drop the old column
ALTER TABLE
Merchant_Pending_Functions
DROP COLUMN
NumberOfLocations_old
Make sure you add the data_type of the column to modify.
ALTER TABLE TABLE_NAME MODIFY COLUMN_NAME DATA_TYPE NULL;