I have an Android app with SQLite and use the following line when creating the table:
_id integer primary key autoincrement
My question is, can I do an update on this row and change the _id if my user wants to? So, for example, I'll have two records in the DB:
1 My First Record
2 My Second Record
Now after my update, I'd like something like:
1 My First Record
38 My Second Record
Will SQLite allow this and easily adapt? i.e. the next primary key autocreated will be #39?
As documented, changing the _id is allowed and will update the autoincrement sequence properly.
Just try it:
$ sqlite3
SQLite version 3.7.16 2013-01-11 09:58:54
Enter ".help" for instructions
Enter SQL statements terminated with a ";"
sqlite> CREATE TABLE MyTable(_id INTEGER PRIMARY KEY AUTOINCREMENT);
sqlite> INSERT INTO MyTable DEFAULT VALUES;
sqlite> INSERT INTO MyTable DEFAULT VALUES;
sqlite> UPDATE MyTable SET _id = 38 WHERE _id = 2;
sqlite> INSERT INTO MyTable DEFAULT VALUES;
sqlite> SELECT * FROM MyTable;
1
38
39
if you want to do such things, you'll have to manage the autoincrement by yourself. SQLite will not allow you to change the value of autogenerated field. Another approach is to add another field and use it instead.
Also, if there is some kind of workaround, I can assure you it's not a good practice.
You should never modify primary keys this way, e.g. think of foreign keys etc. Sqlite won't allow this.
Apart from that, the auto increment counter is stored in a seperate table and has no idea what you are doing. Even if it was allowed to change the primary key, the counter would remain unaffected.
Related
I want to delete all my values in my sqlite Table.
When I click my button it deletes everything besides the id it keeps counting
so for example:
ID, FIRSTNAME, LASTNAME
1, Jack Sparrow
2, Johhny Dep
if I press delete and add new values, it shows this
ID, FIRSTNAME, LASTNAME
2, Obama Barack
3, Mike Tyson
this is my method
private void DeleteEverything()
{
SQLiteDatabase db = mDbHelper.getWritableDatabase();
db.execSQL("delete from "+NamesContract.NameEntry.TABLE_NAME);
}
You should consider to drop and recreate the table.
You can find an example here
If you specify ?? INTEGER PRIMARY KEY or ?? INTEGER PRIMARY KEY AUTOINCREMENT then SQlite treats this in a specific way. That is ?? (by the looks of it ID in your case) is an alias for SQLite's rowid.
rowid if not specified when inserting a row will automatically be generated and is in general guaranteed to be unique and is normally incremented by one. However, it is not guaranteed to be incremented nor incremented by one.
If you specify AUTOINCREMENT (i.e. the latter of the two above) then the guarantee, is that a new rowid will be greater but not necessarily by 1.
There is a limit of 9223372036854775807 on the value of rowid. If AUTOINCREMENT is not used and this limit has been reached then an attempt will made to utilise free numbers (e.g. the rowid of rows that have been deleted). If AUTOINCREMENT is specified and the limit has been reached then an insert will fail with an SQLITE_FULL error.
As such, in your case the freed ID's from deleting rows will not be reused.
In short you should never rely upon the rowid (or an alias of it) column being a specific value, rather you should rely upon it just being a unique value purely for the purpose of uniquely identifying a row (and perhaps the fastest way of accessing a row).
You can, albeit it inadvisable, set rowid either by say INSERT INTO mytable (rowid, myothercolumn) VALUES(1, 'other data') or if ID has been used as an alias then INSERT INTO mytable (ID, myothercolumn)VALUES(1, 'other data').
If you really need the first row to be 1 and then 2 and so on then you could DROP the table and then recreate it rather then deleting all rows. e.g. DROP TABLE mytable, followed by CREATE TABLE mytable (ID INTEGER PRIMARY KEY, FIRSTNAME TEXT, LASTNAME TEXT). However, I'd suggest you will just end up with issues which will subsequently be far more complicated to determine and correct.
If you think about it, really what does Johnny Depp being 2 signify rather than Barack Obama being 2? If it is, for example, a popularity rating the surely you'd have some means of determining the popularity and input that directly rather than input it somewhere else to then be used to determine the insertion order and then do the inserts.
Much of the above is basically a summary of SQLite Autoincrement
As an example of unpredictability, a table was created with:-
CREATE TABLE mytable (ID INTEGER PRIMARY KEY, FIRSTNAME TEXT, LASTNAME TEXT)
A row was inserted using INSERT INTO mytable (ID, FIRSTNAME, LASTNAME) VALUES(9223372036854776000,'FRED','BLOGGS'). Note the use of 9223372036854776000 forces above the limit processing.
This was then followed by a number of INSERT INTO mytable(FIRSTNAME, LASTNAME) VALUES('????','????') inserts. Note! without ID, so using SQLITE's unique identifer determination (above the limit processinmg). The resultant table :-
MARY QUITECONTRARY was the 2nd row inserted,TOM SMITH the 3rd. The two RICHARD BURTON rows are an example of where the unique id could be essential for determining a specific row, both were also inserted without specifying the insertion order.
Note! if the above were tried but with AUTOINCREMENT specified, then the second insert, and any subsequent inserts, would fail with an SQLITE_FULL error.
I am using SQLite Database for my application. I have 4 columns- Student_Name,Student_Enroll, Student_Mob, Student_Address in my database. Now I can add new record if and only if one of four column value is different or all values are different. If all column values are same then no new record should be generated.
Can you please guide me to solve this issue?
To enforce that a set of columns must be unique, add a UNIQUE constraint:
create table Students (
/* ID INTEGER PRIMARY KEY, */
Student_Name TEXT,
Student_Enroll TEXT,
Student_Mob TEXT,
Student_Address TEXT,
UNIQUE (Student_Name, Student_Enroll, Student_Mob, Student_Address)
);
This allows new rows only if at least one of the four columns has a different value.
With a plain INSERT, attempting to insert a duplicate row will result in an error. If you simply want to ignore it instead, use INSERT OR IGNORE:
INSERT OR IGNORE INTO Students ...;
Despite of set your column as UNIQUE you also need to resolve the conflict created on each column when you try to insert new data.
To do so, define the behavior to solve the conflict:
"CREATE TABLE table (your columns here...(UNIQUE unique colums here...) ON CONFLICT REPLACE);"
During Create Database line insert UNIQUE ...for each column to insert only unique record.
Solution 1: (Simple)
Define all columns as unique:
create table TableName (id integer primary key autoincrement,
Student_Name text not null unique,
Student_Enroll text not null unique,
Student_Mob text not null unique);
You can add Student_Address as well, if you need to
Solution 2: (bit complex)
Use AND Operator with WHERE clause
INSERT INTO TableName (Student_Name, Student_Enroll, Student_Mob)
SELECT varStudentName, varStudentEnroll, varStudentMob
WHERE NOT EXISTS(SELECT 1 FROM TableName WHERE Student_Name = varStudentName OR Student_Enroll = varStudentEnroll OR Student_Mob = varStudentMob );
//If a record already contains a row, then the insert operation will be ignored.
You can find more information at the sqlite manual.
Live Example:
Open SQLite Online
Paste following code:
INSERT INTO demo (id,name,hint)
SELECT 4, 'jQuery', 'is a cross-platform JavaScript library designed to simplify the client-side scripting of HTML'
WHERE NOT EXISTS(SELECT 1 FROM demo WHERE name = 'jQuery' OR hint = 'is a cross-platform JavaScript library designed to simplify the client-side scripting of HTML' );
SELECT * from demo
Hit RUN
This won't insert 4th record and if you modify both values of WHERE clause then record will be inserted.
This may be a simple question as I have not done any database work for a while!
I have two tables with data like the below
Table 1
Rows with Primary Keys 1,2
Table 2
Rows with Foreign keys 1,2,3,4
I was to be able to perform a DELETE statement which will remove all rows from Table 2 that do not have a corresponding primary key in table 1, which in this case would result in only rows with foreign keys 1 & 2 being left in the table.
I should mention that this is on Android so I am using SQLite and also I am interested in the ease of doing this via a content provider.
Thanks for any help
Try this:
String SQL="DELETE FROM Table2
WHERE (Table2.FQ1,Table2.FQ2) NOT IN (SELECT PK1,PK2 FROM Table1)";
db.SQL(SQL);
But i'm not sure that the (Table2.FQ1,Table2.FQ2) sentence will run into the NOT IN
For a simple android app I'm creating as a teaching tool for myself (for using relational dbs/SQL among other things - pardon the simplicity of the question if you will). I'm pre-creating a sqlite db to ship with the application. I'm doing this based on the following SO question.
I've got two tables with a many to many relationship and a junction table to define those relationships as follows:
CREATE TABLE Names (_id INTEGER PRIMARY KEY,
name TEXT
);
CREATE TABLE Categories (_id INTEGER PRIMARY KEY,
category TEXT
);
CREATE TABLE Name_Category (name_id INTEGER,
category_id INTEGER,
PRIMARY KEY (name_id, category_id),
foreign key (name_id) references Names(_id),
foreign key (category_id) references Categories(_id)
);
I've got sets of insert statements to fill the Names and Categories tables. I'm now faced with the task of filling the junction table. I'm sure that I could create the insert statements by hand by looking up the ids of the names and categories that I want to match, but that seems a bit silly.
In order to automatically create the insert statements for the junction table, I imagine that I could create a script based on a set of name and category pairs that will search for the appropriate ids and dump an insert statement. (I came up with this as I was asking the question and will research it. Don't you love it when that happens?)
Does anybody have any suggestions for ways to do this?
EDIT I added the foreign keys because, as pointed out below, they'll help maintain integrity between the tables.
EDIT #2 To solve this, I created a simple Perl script that would take a text file with name - category pairs and dump them out into another file with the appropriate SQL statements.
The name - category text file has a format as follows:
'Name' 'Category'
The Perl script looks like this:
use strict;
use warnings;
open (my $name_category_pair_file, "<", "name_category.txt") or die "Can't open name_category.txt: $!";
open (my $output_sql_file, ">", "load_name_category_junction_table.sqlite") or die "Can't open load_name_category_junction_table.sqlite: $!";
while (<$name_category_pair_file>) {
if (/('[a-zA-Z ]*') ('[a-zA-Z ]*')/) {
my $sql_statement = "INSERT INTO Name_Category VALUES (
(SELECT _id FROM Names WHERE name = $1),
(SELECT _id FROM Categories WHERE category = $2))\;\n\n";
print $output_sql_file $sql_statement;
}
}
close $name_category_pair_file or die "$name_category_pair_file: $!";
close $output_sql_file or die "$output_sql_file: $!";
You can use this insert in your script or code (replacing the strings or using ?):
insert into Name_Category values(
(select _id from Categories where category='CAT1'),
(select _id from Names where name='NAME1'));
Also, you can alter the Name_Category table to constraint on the values that can be inserted and/or deleted:
CREATE TABLE Name_Category ( name_id INTEGER NOT NULL,
category_id INTEGER NOT NULL,
PRIMARY KEY (name_id, category_id),
foreign key (name_id) references Names(_id),
foreign key (category_id) references Categories(_id));
create two main tables first and then create a junction table in which primary key of both main tables will be available as foreign key.. Primary key of junction table will be union
of primary key of first and second main table.
Create trigger now to automatically insert into junction table...
Also don't forget to create table with cascade deletion and cascade updatation so that any value updated or deleted in main tables will be automatically reflected in junction table
I have a table with a composite primary key and I am having trouble inserting. The code used to create the table is:
CREATE TABLE ClassEvent (
EventName varchar(10) NOT NULL,
CourseId varchar(10) NOT NULL,
EventType varchar(20),
EventWeight number(3),
DueDate DATE NOT NULL,
FOREIGN KEY (CourseId) REFERENCES Courses(CourseId),
PRIMARY KEY (CourseId, EventName));
The problem I am having is when I want to insert records that have values that may not be unique for the columns CourseId or EventName, but are a unique combination of the 2.
for example, if I try to run the following 2 inserts:
INSERT INTO ClassEvent VALUES('Assignment 1','60-415','Assignment',10,'12/10/2010');
INSERT INTO ClassEvent VALUES('Project 1','60-415','Project',15,'5/12/2010');
I get the following error:
Error: columns CourseId, EventName are not unique.
and the second insert does not make it into the DB. Why does this error out? I thought that a composite primary key requires that the combination of both values are unique. In my above inserts, the values for the EventName column are different even though the values for CourseId are the same. Shouldn't this be seen as 2 unique combinations and thus 2 different primary keys?
My table needs to be able to hold several different events for each CourseId, but each Event must be unique for each Course. I need to be able to insert values into the table like:
EventName CourseId
Assignment 1 60-415
Project 1 60-415
Assignment2 60-415
Project 2 60-415
Assignment 1 60-367
Project 1 60-367
and so on. Can anyone tell me how I can get this to work? Why are these composite PK's not being seen as unique entries? Any help would be much appreciated.
Here is the java function I am using for the insert:
public void addNewClassEvent(ContentValues values) {
SQLiteDatabase db = openConnection();
db.insert("ClassEvent", null, values);
db.close();
}
Could this be causing the problem?
You can have a composite primary key in SQLite, but you
have to create the key when you create the table:
CREATE TABLE example1(
field1 FLOAT,
field2 TEXT,
PRIMARY KEY(field1, field2)
);
You cannot create the primary key after the fact using ALTER TABLE.
On the other hand, you can create a UNIQUE INDEX after the fact
which has essentially the same effect as a PRIMARY KEY:
CREATE UNIQUE INDEX pk_index ON "table1"("field1","field2");
I am not sure how you have created, the tables, and if you have added the primary index later, but grab the database to your desktop, and check out how works in a desktop environment.
You can't make combinations like that, but you don't need them. What is stopping you from just having a truly id column ?