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 a value in informix which is like this :
value AMOUNT: <15000000.00> USD
I need to fetch 15000000.00 afrom the above.
I am using this query to fetch the data between <> as workaround
select substring (value[15,40]
from 1 for length (value[15,40]) -5 )
from tablename p where value like 'AMOUNT%';
But, this is not generic as the lenght may vary.
Please help me with a generic query for this, fetch the data between <>.
The database I am using is Informix version 9.4.
It's a diabolical problem, created by whoever chose to break one of the fundamental rules of database design: that the content of a column should be a single, indivisible value.
The best solution would be to modify the table to contain a value_descr = "AMOUNT", a value = 15000000.00, and a value_type = "USD", and ensure that the incoming data is stored in that fashion. Easier said than done, I know.
Failing that, you'll have to write a UDR that parses the string and returns the numeric portion of it. This would be feasible in SPL, but probably very slow. Something along the lines of:
CREATE PROCEDURE extract_value (inp VARCHAR(255)) RETURNING DECIMAL;
DEFINE s SMALLINT;
DEFINE l SMALLINT;
DEFINE i SMALLINT;
FOR i = 1 TO LENGTH(inp)
IF SUBSTR(inp, i, 1) = "<" THEN
LET s = i + 1;
ELIF SUBSTR(inp, i, 1) = ">" THEN
LET l = i - s - 1;
RETURN SUBSTR(inp, s, l)::DECIMAL;
END IF;
END FOR;
RETURN NULL::DECIMAL; -- could not parse out number
END PROCEDURE;
... which you would execute thus:
SELECT extract_value(p.value)
FROM tablename AS p
WHERE p.value LIKE 'AMOUNT%'
NB: that procedure compiles and produces output in my limited testing on version 11.5. There is no validation done to ensure the string between the <> parses as a number. I don't have an instance of 9.4 handy, but I haven't used any features not available in 9.4 TTBOMK.
I'm trying to develop a blog using webpy.
def getThread(self,num):
myvar = dict(numero=num)
print myvar
que = self.datab.select('contenidos',vars=myvar,what='contentTitle,content,update',where="category LIKE %%s%" %numero)
return que
I've used some of the tips you answer in this web but I only get a
<type 'exceptions.NameError'> at /
global name 'numero' is not defined
Python C:\xampp\htdocs\webpy\functions.py in getThread, line 42
Web GET http://:8080/
...
I'm trying to make a selection of some categorized posts. There is a table with category name and id. There is a column in the content table which takes a string which will be formatted '1,2,3,5'.
Then the way I think I can select the correct entries with the LIKE statement and some %something% magic. But I have this problem.
I call the code from the .py file which builds the web, the import statement works properly
getThread is defined inside this class:
class categoria(object):
def __init__(self,datab,nombre):
self.nombre = nombre
self.datab = datab
self.n = str(self.getCat()) #making the integer to be a string
self.thread = self.getThread(self.n)
return self.thread
def getCat(self):
'''
returns the id of the categorie (integer)
'''
return self.datab.select('categorias',what='catId', where='catName = %r' %(self.nombre), limit=1)
Please check the correct syntax for db.select (http://webpy.org/cookbook/select), you should not format query with "%" because it makes code vulnerable to sql injections. Instead, put vars in dict and refer to them with $ in your query.
myvars = dict(category=1)
db.select('contenidos', what='contentTitle,content,`update`', where="category LIKE '%'+$category+'%'", vars=myvars)
Will produce this query:
SELECT contentTitle,content,`update` FROM contenidos WHERE category LIKE '%'+1+'%'
Note that I backquoted update because it is reserved word in SQL.
I want to update the Interest field in my database. My SQL query is like as per below
Update Table_Name set Interest = Principal * Power(( 1 + (rate /
100),year)
This query works fine in MySQL but don't work with SQLite.
The error says that No Power funcation found
Does anyone know how to resolve this problem as I have to do this using query to update more than 3000 records at a time.
SQLite doesn't have a lot of functions available. But the good news is that is easy enough to add your own.
Here's how to do it using the C API (which also works from Objective-C code).
First write a power function:
void sqlite_power(sqlite3_context *context, int argc, sqlite3_value **argv) {
double num = sqlite3_value_double(argv[0]); // get the first arg to the function
double exp = sqlite3_value_double(argv[1]); // get the second arg
double res = pow(num, exp); // calculate the result
sqlite3_result_double(context, res); // save the result
}
Then you need to register the function:
int res = sqlite3_create_function(dbRef, "POWER", 2, SQLITE_UTF8, NULL, &sqlite_power, NULL, NULL);
The 2 is the number of arguments for the function. dbRef is of course the sqlite3 * database reference.
You can also create an SQLite user-defined function from python. Based on the example at docs.python.org: sqlite3.Connection.create_function
Create a python function:
def sqlite_power(x,n):
return int(x)**n
print(sqlite_power(2,3))
# 8
Create a SQLite user-defined function based on the python function:
con = sqlite3.connect(":memory:")
con.create_function("power", 2, sqlite_power)
Use it:
cur = con.cursor()
cur.execute("select power(?,?)", (2,3))
print cur.fetchone()[0]
# 8
I was strugginling with this too, but if all you need is powers of 2 (or multiples, etc) there is a simpler way:
Use the shift operator, e.g
SELECT 1 << mytable.value
SELECT 1 << (table.x + etc..)
https://www.cafe-encounter.net/p3244/installing-and-using-sqlite-extensions-on-macos-and-maybe-windows-linux-too
Step was to build the Math extensions library that some wonderful person named Liam Healy wrote:
Enter following command in terminal :
Step 1) Download/ Open link http://sqlite.org/contrib/download/extension-functions.c?get=25
Step 2) Go to location where extension-functions.c is downloaded. Run command "gcc -fno-common -dynamiclib extension-functions.c -o libsqlitefunctions.dylib". This will create file libsqlitefunctions.dylib at same place then you can use that in your ios application from xcode.
Now in your cocoa app you can add:
“SELECT load_extension(’libsqlitefunctions.dylib’);”
and then you have access to all kinds of glorious methods like COS, SQRT, etc! You can use them in your app like this:
//Activate database loading
sqlite3_enable_load_extension(database, 1);
sqlite3_load_extension(database,”libsqlitefunctions.dylib”,0,0);
If you are using SQLite NuGet package in a .NET project, you can write an extension method and bind it at runtime;
[SQLiteFunction("pow", 2, FunctionType.Scalar)]
public class SQLitePowerExtension : SQLiteFunction
{
public override object Invoke(object[] args)
{
double num = (double)args[0];
double exp = (double)args[1];
return Math.Pow(num, exp);
}
}
And then use it like this;
using (var conn = new SQLiteConnection("Data Source=:memory:"))
{
conn.Open();
conn.BindFunction(typeof(SQLitePowerExtension).GetCustomAttribute<SQLiteFunctionAttribute>(), new SQLitePowerExtension());
var comm = new SQLiteCommand("CREATE TABLE test (num REAL, exp REAL, result REAL)", conn);
comm.ExecuteNonQuery();
// Populate with some data - not shown
comm = new SQLiteCommand($"UPDATE test SET result = pow(num, exp))", conn);
comm.ExecuteNonQuery();
}
This could be solved with SQL. This works for integer exponents:
Drop Table if Exists args ;
Create Table args as Select 2.5 as Base, 4 as Exponent ;
WITH RECURSIVE pow(exponent, exponent_remainder, base, result) as (
--FIRST EXPRESSION
SELECT exponent,exponent -1 , base,base
FROM args
union all
--SECOND EXPRESSION
select Args.exponent,pow.exponent_remainder -1, pow.base,pow.result * pow.base
from args
join pow on args.exponent = pow.exponent
where pow.exponent_remainder >= 0
)
select pow.result
from pow
where pow.exponent_remainder = 0;
Actually sqlite does have pow/power as a built-in mathematical function, but you need to enable it with DSQLITE_ENABLE_MATH_FUNCTIONS.
From https://www.sqlite.org/lang_mathfunc.html#pow:
The math functions shown below are part of the SQLite amalgamation source file but are only active if the amalgamation is compiled using the -DSQLITE_ENABLE_MATH_FUNCTIONS compile-time option.
SQLite doesn't provide a power function or operator. You'll have to implement it yourself via sqlite3_create_function….