I have researched how to drop a column using SQLite. Due to the lack of a DROP COLUMN statement, I am considering using the following workaround:
Delete column from SQLite table
This involves moving all data into a temporary table, dropping the original, and then re-creating it.
If I have a cascading delete dependency on the table I want to modify, how can I prevent any dependent tables from losing data?
Ex.
CREATE TABLE A (
id INTEGER PRIMARY KEY,
name TEXT,
dummy INTEGER
)
CREATE TABLE B (
id INTEGER PRIMARY KEY,
name TEXT,
a_id INTEGER,
FOREIGN KEY (a_id) REFERENCES A(id) ON DELETE CASCADE
)
Let's say I want to remove Column "dummy" from Table A, but I don't want to affect any rows in Table B. Can this be done?
Foreign key constraints can be disabled with a PRAGMA.
Just execute PRAGMA foreign_keys = off before removing records.
Cascading Deletes are based on records (rows) not attributes (columns). Removing the dummy column will not cause any cascading deletes as you are not removing any records from the parent table.
Related
I have around 20 mapping tables which refer to a single table.
The single table being referenced is,
field (
id integer,
value char
)
The mapping tables are as,
employee_field_map (
employee_id integer references employee(id),
field_id references field(id)
)
dept_field_map (
dept_id integer references dept(id),
field_id references field(id)
)
and similar additional 18 mapping tables.
Now if I want to delete number of records from the field table where field.id = employee_field_map.field_id it takes very long amount of time because there are 20 mapping tables which refer to the field table; And for each of that mapping table a constraint violation check is performed before deleting a record from the field table.
A field table rcord will always be referenced by only one of the mapping table at a time.
In above scenario before deleting a record from field table of course the corresponding record in employee_field_map table is deleted first. So I know for sure that none of the mapping table contains a reference to the field table record being deleted. So is there a way to tell the database engine not to perform those constraint checks when the delete on field table is being performed?
Disabling the constraints is not an option unfortunately. Please advise.
Assuming each of the mapping tables has an index on field_id, then the lookups should not be expensive.
I am wondering why you are not declaring them using cascading delete foreign key references:
employee_field_map (
employee_id integer references employee(id),
field_id references field(id) on delete cascade
);
Nothing in your data model is saying that the field is in only one mapping table. In fact, I don't see why things are broken out the way they are. Presumably there is a reason for breaking the fields apart like this instead of just having a "type" column in the fields table.
I need to create a table having a field, which is a foreign key referencing to another query rather than existing table. E.g. the following statement is correct:
CREATE TABLE T1 (ID1 varchar(255) references Types)
but this one throws a syntax error:
CREATE TABLE T2 (ID2 varchar(255) references SELECT ID FROM BaseTypes UNION SELECT ID FROM Types)
I cannot figure out how I can achieve my goal. In the case it’s needed to introduce a temporary table, how can I force this table being updated each time when tables BaseTypes and Types are changed?
I am using Firebird DB and IBExpert management tool.
A foreign key constraint (references) can only reference a table (or more specifically columns in the primary or unique key of a table). You can't use it to reference a select.
If you want to do that, you need to use a CHECK constraint, but that constraint would only be checked on insert and updates: it wouldn't prevent other changes (eg to the tables in your select) from making the constraint invalid while the data is at rest. This means that at insert time the value could meet the constraint, but the constraint could - unnoticed! - become invalid. You would only notice this when updating the row.
An example of the CHECK-constraint could be:
CREATE TABLE T2 (
ID2 varchar(255) check (exists(
SELECT ID FROM BaseTypes WHERE BaseTypes.ID = ID2
UNION
SELECT ID FROM Types WHERE Types.ID = ID2))
)
For a working example, see this fiddle.
Alternatively, if your goal is to 'unite' two tables, define a 'super'-table that contains the primary keys of both tables, and reference that table from the foreign key constraint. You could populate and update (eg insert and delete) this table using triggers. Or you could use a single table, and replace the existing views with an updatable view (if this is possible depends on the exact data, eg IDs shouldn't overlap).
This is more complex, but would give you the benefit that the foreign key is also enforced 'at rest'.
I have a table which is referenced by multiple tables (around 52) and further,few of the child tables have multiple foreign keys also that is referencing other tables too.
I want to delete a record from parent table, I am unable to do so, as I am getting error "The DELETE statement conflicted with the REFERENCE constraint "FK_xxx". The conflict occurred in database "MyDB", table "dbo.A", column 'x'."
I want a generalized T-SQL solution which is irrespective of tables and number of references.
You have to look at the "on delete" keyword which is a part of the foreign key constraint definition.
Basically you have 4 options:
NO ACTION (does nothing)
CASCADE (deletes the child aswell)
SET NULL (sets the reference field to null)
SET DEFAULT (sets the reference field to the default value)
An example would be:
CREATE TABLE parent (
id INT NOT NULL,
PRIMARY KEY (id)
) ENGINE=INNODB;
CREATE TABLE child (
id INT,
parent_id INT,
INDEX par_ind (parent_id),
FOREIGN KEY (parent_id)
REFERENCES parent(id)
ON DELETE CASCADE -- replace CASCADE with your choice
) ENGINE=INNODB;
(for this example and more details look here: http://dev.mysql.com/doc/refman/5.7/en/create-table-foreign-keys.html )
If you now want to modify your constraint, you first have to drop it, and create a new one like for example:
ALTER TABLE child
ADD CONSTRAINT fk_name
FOREIGN KEY (parent_id)
REFERENCES parent(id)
ON DELETE CASCADE; -- replace CASCADE with your choice
I hope this helped. Also to mention it, you should think about maybe not really deleting your parent, and instead creating another boolean column "deleted", which you fill with "yes" if someone clicks the delete. In the "Select"-query you filter then by that "deleted" column.
The advantage is, that you do not lose the history of this entry.
Your problem is this: A FK constraint is designed to prevent you from creating an orphaned child record in any of the 52 tables. I can provide you with the script you seek, but you must realise first that when you try to re-enable the FK constraints the constraints will fail to re-enable because of the orphaned data (which the FK constraints are designed to prevent). For your next step, will have to delete the orphaned data in each of the 52 tables first anyway. It is actually much easier just to redo the constraints with ON DELETE CASCADE, or drop the constraints and forget about referential integrity altogether. You can't have it both ways.
Background Information
For some reason, while inserting a huge data into multiple tables from xml, my primary keys are shifted by an offset.. (Maybe because of multiple failed attempts :P)
I have two tables.. tableA and tableB. They are in one-to-may relationship.
tableA is the parent table and has the Primary key column ...say DataIndex.
now DataIndex has come out like this..
685, 686, 687... and so on.
and corresponding values present in the child table i.e TableB are the same.
Problem
How do I shift the values up so that DataIndex start from 1, 2, 3..and so on; In both tables ?
I'm assuming that the primary key is actually an identity column that auto-increments itself upon insertion. What you will need to do is 'reseed' the identity column. You can do this by renaming the table, create a duplicate table with the original name, then inserting the data from the old table to the new one (the primary key field will be reset and will auto-increment from 1 again). When doing the insertion, make sure to include the old primary key value as an additional column for reference in the other tables.
To match up the related table, all you will need to do is do an UPDATE and join to your new table on the old primary key value:
UPDATE tableB SET
PRIMARYKEYCOLUMN = tableA.PRIMARYKEYCOLUMN
FROM tableA
WHERE
tableA.OLDPRIMARYKEYCOLUMN = tableB.PRIMARYKEYCOLUMN
I would do the following:
construct a list of Foreign Keys pointing to your PK;
ALTER all FKs, adding ON UPDATE CASCADE clause. This step might not work for some databases, you might need to DROP and CREATE constraint again;
Find the smallest current PK values, like: SELECT min(id) FROM tableA;
Refresh PK values: UPDATE tableA SET id = id - min_id + 1;
Remove ON UPDATE CASCADE clause.
Note, that depending on the tableA size and the database engine you're using, it might be faster and easier to completely rebuild the table to avoid bloat of data files.
Remove PK
Walk through records eith DB CURSOR or write a script on any language sequentally reading and changing id's
Restore PK
Setup correct identity seed for the PK
Changing seed might involve identity column drop and recreate.
This isn't a big deal, but my OCD is acting up with the following problem in the database I'm creating. I'm not used to working with databases, but the data has to be stored somewhere...
Problem
I have two tables A and B.
One of the datafields is common to both tables - segments. There's a finite number of segments, and I want to write queries that connect values from A to B through their segment values, very much asif the following table structure was used:
However, as you can see the table Segments is empty. There's nothing more I want to put into that table, rather than the ID to give other table as foreign keys. I want my tables to be as simple as possible, and therefore adding another one just seems wrong.
Note also that one of these tables (A, say) is actually master, in the sense that you should be able to put any value for segment into A, but B one should first check with A before inserting.
EDIT
I tried one of the answers below:
create table A(
id int primary key identity,
segment int not null
)
create table B(
id integer primary key identity,
segment int not null
)
--Andomar's suggestion
alter table B add constraint FK_B_SegmentID
foreign key (segment) references A(segment)
This produced the following error.
Maybe I was somehow unclear that segments is not-unique in A or B and can appear many times in both tables.
Msg 1776, Level 16, State 0, Line 11 There are no primary or candidate
keys in the referenced table 'A' that match the referencing column
list in the foreign key 'FK_B_SegmentID'. Msg 1750, Level 16, State 0,
Line 11 Could not create constraint. See previous errors.
You can create a foreign key relationship directly from B.SegmentID to A.SegmentID. There's no need for the extra table.
Update: If the SegmentIDs aren't unique in TableA, then you do need the extra table to store the segment IDs, and create foreign key relationships from both tables to this table. This however is not enough to enforce that all segment IDs in TableB also occur in TableA. You could instead use triggers.
You can ensure the segment exists in A with a foreign key:
alter table B add constraint FK_B_SegmentID
foreign key (SegmentID) references A(SegmentID)
To avoid rows in B without a segment at all, make B.SegmentID not nullable:
alter table B alter column SegmentID int not null
There is no need to create a Segments table unless you want to associate extra data with a SegmentID.
As Andomar and Mark Byers wrote, you don't have to create an extra table.
You can also CASCADE UPDATEs or DELETEs on the master. Be very carefull with ON DELETE CASCADE though!
For queries use a JOIN:
SELECT *
FROM A
JOIN B ON a.SegmentID = b.SegmentID
Edit:
You have to add a UNIQUE constraint on segment_id in the "master" table to avoid duplicates there, or else the foreign key is not possible. Like this:
ALTER TABLE A ADD CONSTRAINT UNQ_A_SegmentID UNIQUE (SegmentID);
If I've understood correctly, a given segment cannot be inserted into table B unless it has also been inserted into table A. In which case, table A should reference table Segments and table B should reference table A; it would be implicit that table B ultimately references table Segments (indirectly via table A) so an explicit reference is not required. This could be done using foreign keys (e.g. no triggers required).
Because table A has its own key I assume a given segment_ID can appear in table A more than once, therefore for B to be able to reference the segment_ID value in A then a superkey would need to be defined on the compound of A_ID and segment_ID. Here's a quick sketch:
CREATE TABLE Segments
(
segment_ID INTEGER NOT NULL UNIQUE
);
CREATE TABLE A
(
A_ID INTEGER NOT NULL UNIQUE,
segment_ID INTEGER NOT NULL
REFERENCES Segments (segment_ID),
A_data INTEGER NOT NULL,
UNIQUE (segment_ID, A_ID) -- superkey
);
CREATE TABLE B
(
B_ID INTEGER NOT NULL UNIQUE,
A_ID INTEGER NOT NULL,
segment_ID INTEGER NOT NULL,
FOREIGN KEY (segment_ID, A_ID)
REFERENCES A (segment_ID, A_ID),
B_data INTEGER NOT NULL
);