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.
I have the following method inside my asp.net mvc web application :
var rack = IT.ITRacks.Where(a => !a.Technology.IsDeleted && a.Technology.IsCompleted);
foreach (var r in rack)
{
long? it360id = technology[r.ITRackID];
if (it360resource.ContainsKey(it360id.Value))
{
long? CurrentIT360siteid = it360resource[it360id.Value];
if (CurrentIT360siteid != r.IT360SiteID)
{
r.IT360SiteID = CurrentIT360siteid.Value;
IT.Entry(r).State = EntityState.Modified;
count = count + 1;
}
}
IT.SaveChanges();
}
When I checked SQL Server profiler I noted that EF will generated the following SQL statement:
exec sp_executesql N'update [dbo].[ITSwitches]
set [ModelID] = #0, [Spec] = null, [RackID] = #1, [ConsoleServerID] = null, [Description] = null, [IT360SiteID] = #2, [ConsoleServerPort] = null
where (([SwitchID] = #3) and ([timestamp] = #4))
select [timestamp]
from [dbo].[ITSwitches]
where ##ROWCOUNT > 0 and [SwitchID] = #3',N'#0 int,#1 int,#2 bigint,#3 int,#4 binary(8)',#0=1,#1=539,#2=1502,#3=1484,#4=0x00000000000EDCB2
I can not understand the purpose of having the following section :-
select [timestamp]
from [dbo].[ITSwitches]
where ##ROWCOUNT > 0 and [SwitchID] = #3',N'#0 int,#1 int,#2 bigint,#3 int,#4 binary(8)',#0=1,#1=539,#2=1502,#3=1484,#4=0x00000000000EDCB2
Can anyone advice?
Entity Framework uses timestamps to check whether a row has changed. If the row has changed since the last time EF retrieved it, then it knows it has a concurrency problem.
Here's an explanation:
http://www.remondo.net/entity-framework-concurrency-checking-with-timestamp/
This is because EF (and you) want to update the updated client-side object by the newly generated rowversion value.
First the update is executed. If this succeeds (because the rowversion is still the one you had in the client) a new rowversion is generated by the database and EF retrieves that value. Suppose you'd immediately want to make a second update. That would be impossible if you didn't have the new rowversion.
This happens with all properties that are marked as identity or computed (by DatabaseGenertedOption).
In my database I have:
Row ID - Driver ID - Log ID.
Row ID is unique and auto-increments. What I want is for the Log ID to be unique for each row that has that Driver ID.
For example say a row is inserted with Driver ID 1 I want that row to have a Log ID of 1 but the next time a row is inserted with Driver ID 1 I want it to have a Log ID of 2.
How can I achieve this?
By way for database i am using PHPMyAdmin.
----------------Edit----------------------
This is what i have in my PHP now, but it says:
On the webpage: Incorrect integer value: '' for column 'FinesCost' at row 1
And i dump the variables and get this: string(2) "16" string(2) "16" string(2) "16" so i dont understand why it is saying incorrect integer value and why it is saying they are undefines because they are very clearly defined.
In the PHP error log: [19-Jul-2013 10:44:18 Europe/Minsk] PHP Notice: Undefined variable: FinesCostPost2 in C:\inetpub\wwwroot\hosting\Dan\JWT\drivers-log-send.php on line 336
[19-Jul-2013 10:44:18 Europe/Minsk] PHP Notice: Undefined variable: TravelExpensesPost2 in C:\inetpub\wwwroot\hosting\Dan\JWT\drivers-log-send.php on line 336
///PHP TO INSERT DRIVER'S BANK DETAILS INTO BANK DATABASE
session_start();
$host=""; // Host name
$username=""; // Mysql username
$password=""; // Mysql password
$db_name=""; // Database name
$tbl_name="jwtdriversbank"; // Table name
$un = "";
$usrname = "";
$usrpass = "";
$userID = "";
mysql_connect("$host", "$username", "$password")or die("cannot connect");
mysql_select_db("$db_name")or die("cannot select DB");
if(isset ($_SESSION['usrName']))
{
$usrname = $_SESSION['usrName'];
}
else
{
echo "4";
}
//var_dump ($usrname);
if(isset ($_SESSION['usrPass']))
{
$usrpass = $_SESSION['usrPass'];
}
else
{
echo "5";
}
$sql="SELECT * FROM jwtdrivers WHERE username='$usrname' and password='$usrpass'";
$result=mysql_query($sql);
$rows=mysql_fetch_array($result);
$userID = $rows['id'];
//var_dump ($userID);
if($userID == "")
{
echo "3";
}
else
{
$TotalProfitPost = $TotalProfit;
$LateFeePost = $LateFee;
$FinesCostPost2 = $FinesCost;
$TravelExpensesPost2 = $TravelExpenses;
$FuelCostPost = $FuelCost;
$CargoDamagePost = $CargoDamage;
$TruckDamagePost = $TruckDamage;
var_dump ($TotalProfitPost);
var_dump($FinesCostPost2);
var_dump($TravelExpensesPost2);
$sql="INSERT INTO jwtdriversbank2 (DriverID, LogID, TotalProfit, LateFee, FinesCost, TravelExpenses, FuelCost, CargoDamage, TruckDamage) VALUES ('$userID', COALESCE((Select MAX(LogID) from jwtdriversbank2 tab2 where tab2.DriverID = '$userID'),0)+1,'$TotalProfitPost','$LateFeePost', '$FinesCostPost2' , '$TravelExpensesPost2' ,'$FuelCostPost','$CargoDamagePost','$TruckDamagePost')";
$result = mysql_query($sql);
if($result)
{
}
else
{
die(mysql_error());
}
}
Add a primary key for the two columns.
It should do the trick.
Look at this link for help
ALTER TABLE table_name
ADD CONSTRAINT pk_DriverID PRIMARY KEY (DriverID,LogID)
Do not forget to drop the first primary key because you will not need it no more.
EDIT : COMPLETE WITH THE OTHER ANSWER
Here is the code to insert your data.
Insert into <table_name>
values p_RowID, p_DriverID, COALESCE((Select MAX(Log_id) from <table_name> tab2 where tab2.Driver_id = p_DriverID),0)+1;
That should close the question.
You did not defined variable because PHP can't read them.
I opened your program inside VIM editor and I found "<200c>" char inside $FineCostPost2 in the SQL query. You have to change it to make it work.
A quick solution would be to use a subquery to find the maximum log (last log id) then increment it, something like this
Insert into <table_name>
values p_RowID, p_DriverID, COALESCE((Select MAX(Log_id) from <table_name> tab2 where tab2.Driver_id = p_DriverID),0)+1;
Here p_RowID and p_DriverID are the values you pass to insert into your table. The Coalesce function would check the given value and if it is NULL then it would replace it with the second parameter, in this case 0
I use Sql server 2008 to store my data,and the table structure like that
index float not null,
type int not null,
value int not null,
and the (index,type) is unique.there are not two datas has the same index and the same type.
So when I insert the data to the table, I have to check the (index,type) pair whether in the table already, if it exists I use update statement, otherwise, I insert it directly.but I think this is not a efficient way,because:
Most of the data' index-type pair is not existed int the table.so the select operation is waste, especially the table is huge.
When I use C# or other CLR language to insert the data, I can't use batch copy or batch insert.
is there any way to overwrite the data directly without check whether it is existed in the table?
If you want to update OR insert the data, you need to use merge:
merge MyTable t using (select #index index, #type type, #value value) s on
t.index = s.index
and t.type = s.type
when not matched insert (index, type value) values (s.index, s.type, s.value)
when matched update set value = s.value;
This will look at your values and take the appropriate action.
To do this in C#, you have to use the traditional SqlClient:
SqlConnection conn = new SqlConnection("Data Source=dbserver;Initial Catalog=dbname;Integrated Security=SSPI;");
SqlCommand comm = new SqlCommand();
conn.Open();
comm.Connection = conn;
//Add in your values here
comm.Parameters.AddWithValue("#index", index);
comm.Parameters.AddWithValue("#type", type);
comm.Parameters.AddWithValue("#value", value);
comm.CommandText =
"merge MyTable t using (select #index index, #type type, #value value) s on " +
"t.index = s.index and t.type = s.type " +
"when not matched insert (index, type value) values (s.index, s.type, s.value) " +
"when matched update set value = s.value;"
comm.ExecuteNonQuery();
comm.Dispose();
conn.Close();
conn.Dispose();
You should make (index, type) into a composite primary key (aka compound key).
This would ensure that the table can only even have unique pairs of these (I am assuming the table does not have a primary key already).
If the table does have a primary key, you can add a UNIQUE constraint onto those columns with similar effect.
Once defined, this means that any attempt to insert a duplicate pair would fail.
Other answers recommend constraints. Creating constraints just means you will be executing insert statements that trigger errors. The next step (after having created the constraints) is something like INSERT ON DUPLICATE KEY UPDATE, which apparently does have an Sql Server equivalent.
I have my table structure like:
CREATE TABLE test_two_tabel.T1 ( T1_ID INT NOT NULL AUTO_INCREMENT , A1 INT NULL , B1 VARCHAR(45) NULL , C1 VARCHAR(45) NULL , D1 DATETIME NULL , PRIMARY KEY (T1_ID) );
In Grails:
package twotables
class T1 {
Integer a
String b
String c
Date d
static mapping = {
table "T1"
version false
id column:"T1_ID"
a1 column:"a1"
b1 column:"b1"
c1 column:"c1"
d1 column:"d1"
}
static constraints = {
id()
a1()
b1()
c1()
d1()
}
}
Every time I execute my program... Grails deletes my tables in the DB, does anyone know what's happening?
You need to change value of dbCreate from 'create-drop' to 'update' at grails-app/conf/DataSource.groovy
You current value probably is:
development {
dataSource {
dbCreate = "create-drop" // one of 'create', 'create-drop','update'
url = "***"
}
}
this means that Grails will recreate all tables on every restart. If you'll set this as update it will try to update table structure, according to your data model classes.
You can read more about Grails DB configuration at http://www.grails.org/doc/latest/guide/3.%20Configuration.html#3.3%20The%20DataSource
it could be a few things. As #splix mentioned, it could be the 'create-drop' settings.
Also, if you never changed your datasource, Grails uses an in-memory database, so it only lasts as long as the program runs. You can tell hsqldb to persist to file instead of be in memory. You can also change it to point to something like mysql. Look here.