I've got a function that I'd like use to delete a row in my database. This is the only way I've used the DELETE statement to remove a row before but I want the 1 to be replaced by a variable called recID so that the value of recID is the row ID number which is deleted. So if recID = 6, I want the function to delete the row with ID = 6. I hope that makes sense.
'DELETE FROM MyRecords WHERE ID=1';
The notation I've been using is the following, if it helps or makes any difference.
db.transaction(function(transaction) {
transaction.executeSql( //DELETE STATEMENT HERE );
});
executeSql supports arguments (check definition).
Use it like:
db.transaction(function(transaction) {
transaction.executeSql("DELETE FROM MyRecords WHERE ID=?", [recId]);
});
If you're certain that your variable, recID, will only ever contain numbers, you can just use:
transaction.executeSql("DELETE FROM MyRecords WHERE ID=" + recID);
If recID comes from outside your application (user input) however, it either needs to be sanitized, or use a prepared statement and use the database API to set the parameter after the statement has been prepared. Otherwise you open yourself up to SQL injection attacks.
I don't know the details of your SQLite wrapper, or what version of SQLite it wraps, but creating a prepared statement using the SQLite3 C API would go something like this:
// sqlite3* db = ...
sqlite3_stmt* stmt;
sqlite3_prepare_v2(db, "DELETE FROM MyRecords WHERE ID=?", -1, &stmt, 0);
sqlite3_bind_int(stmt, 1, recID);
sqlite3_step();
// ...
sqlite3_finalize(stmt);
This simple example excludes all the error checking you'd want to do in a real application, but since you're using a wrapper that has different syntax anyway, you'd have to figure out how it wraps these functions anyway.
I am trying to get the primary key of an inserted row within a transaction scope, because I do not want to leave the db in a logically inconsistent state.
My problem is I cannot find a way to retrieve the ID value of a previously executed query, which I want to use for the next insert query. Querying the PostgreSQL database while the transaction is in effect shows no results in the non-foreign-key table(the row is not yet committed?). I believe this is due to the transaction's isolation level.
Below is what I'm trying to do with production code, albeit slightly edited and narrowed down to one function for clarity. const int lastInsertId is always 0, which in this context should mean no value was found (technically that toInt() function failed). I tried manually inserting a valid non-foreign-key row, and then calling LASTVAL() which produced the expected result - the ID of the inserted row.
So, what am I doing wrong? What am I missing or misunderstanding here?
void createEntityWithoutForiegnKeyConstraint(const QString &nameOfEntity)
{
db_.transaction();
QSqlQuery insertQuery(db_);
insertQuery.prepare("INSERT INTO \"EntityWithoutForeignKey\" (\"name\") VALUES (:name);");
insertQuery.bindValue(":name", nameOfEntity);
execQuery(__LINE__, insertQuery);
QSqlQuery lastIdQuery("SELECT LASTVAL();", db_); // auto executes
const int lastInsertId = lastIdQuery.value(0).toInt();
if (lastInsertId <= 0) // 0 is not a valid ID
throw exception("Oh noes.");
createEntityWithForeignKeyConstraint(lastInsertId, someData);
if (!db_.commit())
db_.rollback();
}
I realise this is an old question but in Qt 5.10 (and likely earlier) there is a function QSqlQuery::lastInsertId() which can be called after QSqlQuery::exec().
It's quite useful if you are using a database such as SQLite which doesn't support the RETURNING clause on an INSERT statement.
QSqlQuery::lastInsertId() documentation.
Usage is something along the lines of the following:
QSqlQuery q;
q.prepare("INSERT INTO table_name VALUES(:some_column_name)");
q.bindValue(":some_column_name", "FooBar");
q.exec();
qDebug() << "Last ID was:" << q.lastInsertId();
#Kasheen is perfectly right but I want to set the focus on one important aspect you should keep in mind: Don't forget to encapsulate everything in one single transaction.
Why? It saves time and avoids database corruption if the second insert using the generated primary key (you got with q.lastInsertId()) fails.
So your insert should look like this (this is basically #Kasheen's answer with some additions):
db_.transaction(); // Starts a transaction
QSqlQuery q;
// first insert
q.prepare("INSERT INTO table_name VALUES(:some_column_name)");
q.bindValue(":some_column_name", "FooBar");
q.exec();
auto pk == q.lastInsertId().toInt;
// second insert
q.prepare("INSERT INTO other_table VALUES(:other_column_name, :fk)");
q.bindValue(":other_column_name", "OtherText");
q.bindValue(":fk", pk);
q.exec();
db_.commit(); // Commits transaction
#average joe does the transaction handling correctly but you might forget it if you look on the solution only.
I'm trying to make an insertion from one database called suspension to the table called Notification in the ANimals database. My stored procedure is this:
ALTER PROCEDURE [dbo].[spCreateNotification]
-- Add the parameters for the stored procedure here
#notRecID int,
#notName nvarchar(50),
#notRecStatus nvarchar(1),
#notAdded smalldatetime,
#notByWho int
AS
BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;
-- Insert statements for procedure here
INSERT INTO Animals.dbo.Notification
(
NotRecID,
NotName,
NotRecStatus,
NotAdded,
NotByWho
)
values (#notRecID, #notName, #notRecStatus, #notAdded, #notByWho);
END
The null inserting is to replenish one column that otherwise will not be filled, I've tried different ways, like using also the names for the columns after the name of the table and then only indicate in values the fields I've got. I know it is not a problem of the stored procedure because I executed it from the sql server management studio and it works introducing the parameters. Then I guess the problem must be in the repository when I call the stored procedure:
public void createNotification(Notification not)
{
try
{
DB.spCreateNotification(not.NotRecID, not.NotName, not.NotRecStatus,
(DateTime)not.NotAdded, (int)not.NotByWho);
}
catch
{
return;
}
}
And I call the method here:
public void createNotifications(IList<TemporalNotification> notifications)
{
foreach (var TNot in notifications)
{
var ts = RepositoryService._suspension.getTemporalSuspensionForNotificationID(TNot.TNotRecID);
Notification notification = new Notification();
if (ts.Count != 0)
{
notification.NotName = TNot.TNotName;
notification.NotRecID = TNot.TNotRecID;
notification.NotRecStatus = TNot.TNotRecStatus;
notification.NotAdded = TNot.TNotAdded;
notification.NotByWho = TNot.TNotByWho;
if (TNot.TNotToReplace != 0)
{
var suspensions = RepositoryService._suspension.getSuspensionsAttached((int)TNot.TNotToReplace);
foreach (var sus in suspensions)
{
sus.CtsEndDate = TNot.TNotAdded;
sus.CtsEndNotRecID = TNot.TNotRecID;
DB.spModifySuspensionWhenNotificationIsReplaced((int)TNot.TNotToReplace, (int)sus.CtsEndNotRecID, (DateTime) sus.CtsEndDate);
}
DB.spReplaceNotification((int)TNot.TNotToReplace, DateTime.Now);
createNotification(notification);
}
else
{
createNotification(notification);
}
}
}
deleteTemporalNotifications(notifications);
}
It does not record the value in the database. I've been debugging and getting mad about this, because it works when I execute it manually, but not when I automatize the proccess in my application. Does anyone see anything wrong with my code?
Thank you
EDIT: Added more code. It still doesn't work changing that, I mean, the procedure works if I execute it, so I don't know what could be the error. In fact, I don't get any error. Could it be a matter of writin in a table that is not in the database where you have your stored procedure?
I would specify your column names and DONT incude the NULL at all for that column. Just let SQL Server deal with it.
INSERT INTO Animals.dbo.Notification
(
RecID,
[Name],
RecStatus,
Added,
ByWho
)
values (#notRecID, #notName, #notRecStatus, #notAdded, #notByWho);
Run profiler when you try to run it from the application and see what values it realy is sending. That will tell you if the application is creating the correct exec statment to exec the proc.
Also it may be a permissions problem.
Specify your column names:
INSERT INTO Animals.dbo.Notification
(RecID, Name, RecStatus, Added, ByWho)
VALUES
(#notRecID, #notName, #notRecStatus, #notAdded, #notByWho);
"Could it be a matter of writin in a table that is not in the database where you have your stored procedure?"
That may be the problem. You could try adding the "WITH EXECUTE AS OWNER" clause to your stored procedure so that it executes as the owner of the stored procedure. Or grant write permissions for the executing user to the table.
http://msdn.microsoft.com/en-us/library/ms188354.aspx
ok, I finally found out what noone realized lol. It was a very stupid error but got me really mad till I found the problem. It wasn't a problem of permissions, the problem was that I was not executing the procedure from my application, so where I wrote this:
DB.spCreateNotification(not.NotRecID, not.NotName, not.NotRecStatus,
(DateTime)not.NotAdded, (int)not.NotByWho);
When I had to write:
DB.spCreateNotification(not.NotRecID, not.NotName, not.NotRecStatus,
(DateTime)not.NotAdded, (int)not.NotByWho).Execute();
so as you see I was focusing my efforts in much more complex things and I wasn't even executing it...lol.
Thank you all for your answers anyway:)
How do I, reliably, check in SQLite, whether a particular user table exists?
I am not asking for unreliable ways like checking if a "select *" on the table returned an error or not (is this even a good idea?).
The reason is like this:
In my program, I need to create and then populate some tables if they do not exist already.
If they do already exist, I need to update some tables.
Should I take some other path instead to signal that the tables in question have already been created - say for example, by creating/putting/setting a certain flag in my program initialization/settings file on disk or something?
Or does my approach make sense?
I missed that FAQ entry.
Anyway, for future reference, the complete query is:
SELECT name FROM sqlite_master WHERE type='table' AND name='{table_name}';
Where {table_name} is the name of the table to check.
Documentation section for reference: Database File Format. 2.6. Storage Of The SQL Database Schema
This will return a list of tables with the name specified; that is, the cursor will have a count of 0 (does not exist) or a count of 1 (does exist)
If you're using SQLite version 3.3+ you can easily create a table with:
create table if not exists TableName (col1 typ1, ..., colN typN)
In the same way, you can remove a table only if it exists by using:
drop table if exists TableName
A variation would be to use SELECT COUNT(*) instead of SELECT NAME, i.e.
SELECT count(*) FROM sqlite_master WHERE type='table' AND name='table_name';
This will return 0, if the table doesn't exist, 1 if it does. This is probably useful in your programming since a numerical result is quicker / easier to process. The following illustrates how you would do this in Android using SQLiteDatabase, Cursor, rawQuery with parameters.
boolean tableExists(SQLiteDatabase db, String tableName)
{
if (tableName == null || db == null || !db.isOpen())
{
return false;
}
Cursor cursor = db.rawQuery(
"SELECT COUNT(*) FROM sqlite_master WHERE type = ? AND name = ?",
new String[] {"table", tableName}
);
if (!cursor.moveToFirst())
{
cursor.close();
return false;
}
int count = cursor.getInt(0);
cursor.close();
return count > 0;
}
You could try:
SELECT name FROM sqlite_master WHERE name='table_name'
See (7) How do I list all tables/indices contained in an SQLite database in the SQLite FAQ:
SELECT name FROM sqlite_master
WHERE type='table'
ORDER BY name;
Use:
PRAGMA table_info(your_table_name)
If the resulting table is empty then your_table_name doesn't exist.
Documentation:
PRAGMA schema.table_info(table-name);
This pragma returns one row for each column in the named table. Columns in the result set include the column name, data type, whether or not the column can be NULL, and the default value for the column. The "pk" column in the result set is zero for columns that are not part of the primary key, and is the index of the column in the primary key for columns that are part of the primary key.
The table named in the table_info pragma can also be a view.
Example output:
cid|name|type|notnull|dflt_value|pk
0|id|INTEGER|0||1
1|json|JSON|0||0
2|name|TEXT|0||0
SQLite table names are case insensitive, but comparison is case sensitive by default. To make this work properly in all cases you need to add COLLATE NOCASE.
SELECT name FROM sqlite_master WHERE type='table' AND name='table_name' COLLATE NOCASE
If you are getting a "table already exists" error, make changes in the SQL string as below:
CREATE table IF NOT EXISTS table_name (para1,para2);
This way you can avoid the exceptions.
If you're using fmdb, I think you can just import FMDatabaseAdditions and use the bool function:
[yourfmdbDatabase tableExists:tableName].
The following code returns 1 if the table exists or 0 if the table does not exist.
SELECT CASE WHEN tbl_name = "name" THEN 1 ELSE 0 END FROM sqlite_master WHERE tbl_name = "name" AND type = "table"
Note that to check whether a table exists in the TEMP database, you must use sqlite_temp_master instead of sqlite_master:
SELECT name FROM sqlite_temp_master WHERE type='table' AND name='table_name';
Here's the function that I used:
Given an SQLDatabase Object = db
public boolean exists(String table) {
try {
db.query("SELECT * FROM " + table);
return true;
} catch (SQLException e) {
return false;
}
}
Use this code:
SELECT name FROM sqlite_master WHERE type='table' AND name='yourTableName';
If the returned array count is equal to 1 it means the table exists. Otherwise it does not exist.
class CPhoenixDatabase():
def __init__(self, dbname):
self.dbname = dbname
self.conn = sqlite3.connect(dbname)
def is_table(self, table_name):
""" This method seems to be working now"""
query = "SELECT name from sqlite_master WHERE type='table' AND name='{" + table_name + "}';"
cursor = self.conn.execute(query)
result = cursor.fetchone()
if result == None:
return False
else:
return True
Note: This is working now on my Mac with Python 3.7.1
You can write the following query to check the table existance.
SELECT name FROM sqlite_master WHERE name='table_name'
Here 'table_name' is your table name what you created. For example
CREATE TABLE IF NOT EXISTS country(country_id INTEGER PRIMARY KEY AUTOINCREMENT, country_code TEXT, country_name TEXT)"
and check
SELECT name FROM sqlite_master WHERE name='country'
Use
SELECT 1 FROM table LIMIT 1;
to prevent all records from being read.
Using a simple SELECT query is - in my opinion - quite reliable. Most of all it can check table existence in many different database types (SQLite / MySQL).
SELECT 1 FROM table;
It makes sense when you can use other reliable mechanism for determining if the query succeeded (for example, you query a database via QSqlQuery in Qt).
The most reliable way I have found in C# right now, using the latest sqlite-net-pcl nuget package (1.5.231) which is using SQLite 3, is as follows:
var result = database.GetTableInfo(tableName);
if ((result == null) || (result.Count == 0))
{
database.CreateTable<T>(CreateFlags.AllImplicit);
}
The function dbExistsTable() from R DBI package simplifies this problem for R programmers. See the example below:
library(DBI)
con <- dbConnect(RSQLite::SQLite(), ":memory:")
# let us check if table iris exists in the database
dbExistsTable(con, "iris")
### returns FALSE
# now let us create the table iris below,
dbCreateTable(con, "iris", iris)
# Again let us check if the table iris exists in the database,
dbExistsTable(con, "iris")
### returns TRUE
I thought I'd put my 2 cents to this discussion, even if it's rather old one..
This query returns scalar 1 if the table exists and 0 otherwise.
select
case when exists
(select 1 from sqlite_master WHERE type='table' and name = 'your_table')
then 1
else 0
end as TableExists
My preferred approach:
SELECT "name" FROM pragma_table_info("table_name") LIMIT 1;
If you get a row result, the table exists. This is better (for me) then checking with sqlite_master, as it will also check attached and temp databases.
This is my code for SQLite Cordova:
get_columnNames('LastUpdate', function (data) {
if (data.length > 0) { // In data you also have columnNames
console.log("Table full");
}
else {
console.log("Table empty");
}
});
And the other one:
function get_columnNames(tableName, callback) {
myDb.transaction(function (transaction) {
var query_exec = "SELECT name, sql FROM sqlite_master WHERE type='table' AND name ='" + tableName + "'";
transaction.executeSql(query_exec, [], function (tx, results) {
var columnNames = [];
var len = results.rows.length;
if (len>0){
var columnParts = results.rows.item(0).sql.replace(/^[^\(]+\(([^\)]+)\)/g, '$1').split(','); ///// RegEx
for (i in columnParts) {
if (typeof columnParts[i] === 'string')
columnNames.push(columnParts[i].split(" ")[0]);
};
callback(columnNames);
}
else callback(columnNames);
});
});
}
Table exists or not in database in swift
func tableExists(_ tableName:String) -> Bool {
sqlStatement = "SELECT name FROM sqlite_master WHERE type='table' AND name='\(tableName)'"
if sqlite3_prepare_v2(database, sqlStatement,-1, &compiledStatement, nil) == SQLITE_OK {
if sqlite3_step(compiledStatement) == SQLITE_ROW {
return true
}
else {
return false
}
}
else {
return false
}
sqlite3_finalize(compiledStatement)
}
c++ function checks db and all attached databases for existance of table and (optionally) column.
bool exists(sqlite3 *db, string tbl, string col="1")
{
sqlite3_stmt *stmt;
bool b = sqlite3_prepare_v2(db, ("select "+col+" from "+tbl).c_str(),
-1, &stmt, 0) == SQLITE_OK;
sqlite3_finalize(stmt);
return b;
}
Edit: Recently discovered the sqlite3_table_column_metadata function. Hence
bool exists(sqlite3* db,const char *tbl,const char *col=0)
{return sqlite3_table_column_metadata(db,0,tbl,col,0,0,0,0,0)==SQLITE_OK;}
You can also use db metadata to check if the table exists.
DatabaseMetaData md = connection.getMetaData();
ResultSet resultSet = md.getTables(null, null, tableName, null);
if (resultSet.next()) {
return true;
}
If you are running it with the python file and using sqlite3 obviously. Open command prompt or bash whatever you are using use
python3 file_name.py first in which your sql code is written.
Then Run sqlite3 file_name.db.
.table this command will give tables if they exist.
I wanted to add on Diego Vélez answer regarding the PRAGMA statement.
From https://sqlite.org/pragma.html we get some useful functions that can can return information about our database.
Here I quote the following:
For example, information about the columns in an index can be read using the index_info pragma as follows:
PRAGMA index_info('idx52');
Or, the same content can be read using:
SELECT * FROM pragma_index_info('idx52');
The advantage of the table-valued function format is that the query can return just a subset of the PRAGMA columns, can include a WHERE clause, can use aggregate functions, and the table-valued function can be just one of several data sources in a join...
Diego's answer gave PRAGMA table_info(table_name) like an option, but this won't be of much use in your other queries.
So, to answer the OPs question and to improve Diegos answer, you can do
SELECT * FROM pragma_table_info('table_name');
or even better,
SELECT name FROM pragma_table_list('table_name');
if you want to mimic PoorLuzers top-voted answer.
If you deal with Big Table, I made a simple hack with Python and Sqlite and you can make the similar idea with any other language
Step 1: Don't use (if not exists) in your create table command
you may know that this if you run this command that will have an exception if you already created the table before, and want to create it again, but this will lead us to the 2nd step.
Step 2: use try and except (or try and catch for other languages) to handle the last exception
here if you didn't create the table before, the try case will continue, but if you already did, you can put do your process at except case and you will know that you already created the table.
Here is the code:
def create_table():
con = sqlite3.connect("lists.db")
cur = con.cursor()
try:
cur.execute('''CREATE TABLE UNSELECTED(
ID INTEGER PRIMARY KEY)''')
print('the table is created Now')
except sqlite3.OperationalError:
print('you already created the table before')
con.commit()
cur.close()
You can use a simple way, i use this method in C# and Xamarin,
public class LoginService : ILoginService
{
private SQLiteConnection dbconn;
}
in login service class, i have many methods for acces to the data in sqlite, i stored the data into a table, and the login page
it only shows when the user is not logged in.
for this purpose I only need to know if the table exists, in this case if it exists it is because it has data
public int ExisteSesion()
{
var rs = dbconn.GetTableInfo("Sesion");
return rs.Count;
}
if the table does not exist, it only returns a 0, if the table exists it is because it has data and it returns the total number of rows it has.
In the model I have specified the name that the table must receive to ensure its correct operation.
[Table("Sesion")]
public class Sesion
{
[PrimaryKey]
public int Id { get; set; }
public string Token { get; set; }
public string Usuario { get; set; }
}
Look into the "try - throw - catch" construct in C++. Most other programming languages have a similar construct for handling errors.