I wish to check if a record exists and then if it does i want to read the records in the other table. I am using the same cursor of the database that i created but it shows unresolved reference for the cursor inside the if block.
My code:
import psycopg2
conn=psycopg2.connect(host='localhost', database='my_first_db', user='postgres', password='postgres')
curr= conn.cursor()
res=curr.execute("select EXISTS(select * from teachers where t_name='xoxo' AND pass='xoxo2020')")
if curr.fetchone()[0]==1 :
{
curr.execute("select * from students")
result=curr.fetchall()
for x in result:
print(x)
#print('Table exists')
}
else:
print("not found")
print(res)
curr.close()
conn.close()
the curr in the second line of if block shows the unresolved error.
Thanks.
My familiarization with psycopg2 limited so you may have to adjust the following some. But in straight sql this can be accomplished is a single statement:
select *
from students
where exists (select null
from teachers
where t_name = 'xoxo'
and password = 'xoxo2020'
);
As best as I can determine this translates to psycopg2 as (subject ot above):
conn=psycopg2.connect(host='localhost', database='my_first_db', user='postgres', password='postgres')
curr= conn.cursor()
curr.execute("select * from students where exists (select null from teachers where t_name='xoxo' and password='xoxo2020')")
if curr.rowcount > 0
result=curr.fetchall()
for x in result:
print(x)
else:
print("not found")
curr.close()
conn.close()
The main idea is when working with sql stop thinking in terms of
check A; If it exists then do B;
But rather think in terms of
do B where A;
In other words let a single sql statement do ALL the work, including any checking needed.
for me it worked by creating a function outside, for the working inside the if statement and called the function in the if block. and it works fine. Though any answers for the query are welcome.
Related
Using Microsoft Visual FoxPro 9, I have a custom function, "newid()", inside of the stored procedures for Main:
function newId
parameter thisdbf
regional keynm, newkey, cOldSelect, lDone
keynm=padr(upper(thisdbf),50)
cOldSelect=alias()
lDone=.f.
do while not lDone
select keyvalue from main!idkeys where keyname=keynm into array akey
if _tally=0
insert into main!idkeys (keyname) value (keynm)
loop
endif
newkey=akey+1
update main!idkeys set keyvalue=newkey where keyname=keynm and keyvalue=akey
if _tally=1
lDone=.t.
endif
enddo
if not empty(cOldSelect)
select &cOldSelect
else
select 0
endif
return newkey
This function is used to generate a new ID for records added to the database.
It is called as the default value:
I would like to call this newid() function and retrieve its returned value. When executing SELECT newid("TABLENAME"), the error is is thrown:
Invalid subscript reference
How can I call the newid() function and return the newkey in Visual FoxPro 9?
As an addition to what Stefan Wuebbe said,
You actually had your answer in your previous question here that you forgot to update.
From your previous question, as I understand you are coming from a T-SQL background. While in T-SQL (and in SQL generally) there is:
Select < anyVariableOrFunction >
that returns a single column, single row result, in VFP 'select' like that has another meaning:
Select < aliasName >
aliasName is an alias of a working area (or it could be number of a work area) and is used to change the 'current workarea'. When it was used in xBase languages like FoxPro (and dBase), those languages didn't yet meet ANSI-SQL if I am not wrong. Anyway, in VFP there are two Select, this one and SELECT—SQL which definitely requires a FROM clause.
VFP has direct access to variables and function calls though, through the use of = operator.
SELECT newid("TABLENAME")
in T-SQL, would be (you are just displaying the result):
? newid("TABLENAME")
To store it in a variable, you would do something like:
local lnId
lnId = newid("TABLENAME")
* do something with m.lnId
* Note the m. prefix, it is a built-in alias for memory variables
After having said all these, as per your code.
It looks like it has been written by a very old FoxPro programmer and I must admit I am seeing it the first time in my life that someone used "REGIONAL" keyword in VFP. It is from FoxPro 2.x days I know but I didn't see anyone use it up until now :) Anyway, that code doesn't seem to be robust enough in a multiuser environment, you might want to change it. VFP ships with a NewId sample code and below is the slightly modified version that I have been using in many locations and proved to be reliable:
Function NewID
Lparameters tcAlias,tnCount
Local lcAlias, lnOldArea, lcOldReprocess, lcTable, lnTagNo, lnNewValue, lnLastValue, lcOldSetDeleted
lnOldArea = Select()
lnOldReprocess = Set('REPROCESS')
* Uppercase Alias name
lcAlias = Upper(Iif(Parameters() = 0, Alias(), tcAlias))
* Lock reprocess - try once
Set Reprocess To 1
If !Used("IDS")
Use ids In 0
Endif
* If no entry yet create
If !Seek(lcAlias, "Ids", "tablename")
Insert Into ids (tablename, NextID) Values (lcAlias,0)
Endif
* Lock, increment id, unlock, return nextid value
Do While !Rlock('ids')
* Delay before next lock trial
lnStart = Seconds()
Do While Seconds()-lnStart < 0.01
Enddo
Enddo
lnLastValue = ids.NextID
lnNewValue = m.lnLastValue + Evl(m.tnCount,1)
*Try to query primary key tag for lcAlias
lcTable = Iif( Used(lcAlias),Dbf(lcAlias), Iif(File(lcAlias+'.dbf'),lcAlias,''))
lcTable = Evl(m.lcTable,m.lcAlias)
If !Empty(lcTable)
Use (lcTable) In 0 Again Alias '_GetPKKey_'
For m.lnTagNo=1 To Tagcount('','_GetPKKey_')
If Primary(m.lnTagNo,'_GetPKKey_')
m.lcOldSetDeleted = Set("Deleted")
Set Deleted Off
Select '_GetPKKey_'
Set Order To Tag (Tag(m.lnTagNo,'_GetPKKey_')) ;
In '_GetPKKey_' Descending
Locate
lnLastValue = Max(m.lnLastValue, Evaluate(Key(m.lnTagNo,'_GetPKKey_')))
lnNewValue = m.lnLastValue + Evl(m.tnCount,1)
If Upper(m.lcOldSetDeleted) == 'ON'
Set Deleted On
Endif
Exit
Endif
Endfor
Use In '_GetPKKey_'
Select ids
Endif
* Increment
Replace ids.NextID With m.lnNewValue In 'ids'
Unlock In 'ids'
Select (lnOldArea)
Set Reprocess To lnOldReprocess
Return ids.NextID
Endfunc
Note: If you use this, as I see from your code, you would need to change the "id table" name to idkeys, field names to keyname, keyvalue:
ids => idKeys
tablename => keyName
nextId => keyValue
Or in your database just create a new table with this code:
CREATE TABLE ids (TableName c(50), NextId i)
INDEX on TableName TAG TableName
When executing SELECT newid("TABLENAME")
The error: Invalid subscript reference is thrown
The SQL Select command in Vfp requires a From clause.
Running a procedure or a function can, or better usually needs to be done differently:
For example, in the IDE's Command Window you can do a
? newid("xy") && the function must be "in scope",
&& i.e in your case the database that contains the "Stored
&& Procedure" must have been opened in advance
&& or you store the function result in a variable
Local lnNextID
lnNextID = newid("xy")
Or you can use it in an SQL SELECT when you have a From alias
CREATE CURSOR placebo (col1 Int)
INSERT INTO placebo VALUES (8)
Select newid("xy") FROM placebo
Say I have a class variable restemail which stores the email id I need to use to sort out from the select statement in SQLite (Python). Whenever I refer to that variable after my WHERE clause, SQLite treats it as a column and returns an error saying that such a column doesn't exist. Something like this:
restemail=StringVar()
Password=StringVar()
def database(self):
conn = sqlite3.connect('data.db')
with conn:
cursor=conn.cursor()
strrest = self.restemail
cursor.execute('SELECT * FROM Restaurant3 WHERE restemail = strrest')
Can someone tell me how to use a variable inside my SQL queries without it being treated as a column name?
Any help will be appreciated.
Try the sqlite3 variable substitution syntax:
cursor.execute('SELECT * FROM Restaurant3 WHERE restemail = ?', (strrest,))
I have read dozens of similar posts and tried everything but I still get an error message when trying to pass a parameter to a simple query using pyodbc. Apologies if there is an answer to this elsewhere but I cannot find it
I have a very simple table:
select * from Test
yields
a
b
c
This works fine:
import pyodbc
import pandas
connection = pyodbc.connect('DSN=HyperCube SYSTEST',autocommit=True)
result = pandas.read_sql("""select * from Test where value = 'a'""",connection,params=None)
print(result)
result:
value
0 a
However if I try to do the where clause with a parameter it fails
result = pandas.read_sql("""select * from Test where value = ?""",connection,params='a')
yields
Error: ('01S02', '[01S02] Unknown column/parameter value (9001) (SQLPrepare)')
I also tried this
cursor = connection.cursor()
cursor.execute("""select * from Test where value = ?""",['a'])
pyodbcResults = cursor.fetchall()
and still received the same error
Does anyone know what is going on? Could it be an issue with the database I am querying?
PS. I looked at the following post and the syntax there in the first part of answer 9 where dates are passed by strings looks identical to what I am doing
pyodbc the sql contains 0 parameter markers but 1 parameters were supplied' 'hy000'
Thanks
pandas.read_sql(sql, con, index_col=None, coerce_float=True, params=None, parse_dates=None, columns=None, chunksize=None)[https://pandas.pydata.org/pandas-docs/stable/generated/pandas.read_sql.html]¶
params : list, tuple or dict, optional, default: None
example:
cursor.execute("select * from Test where value = %s",['a'])
or Named arguments example:
result = pandas.read_sql(('select * from Test where value = %(par)s'),
db,params={"par":'p'})
in pyodbc write parms directly after sql parameter:
cursor.execute(sql, *parameters)
for example:
onepar = 'a'
cursor.execute("select * from Test where value = ?", onepar)
cursor.execute("select a from tbl where b=? and c=?", x, y)
I'm trying to retrieve some data from a postgresql database using psycogp2, and either exclude a variable number of rows or exclude none.
The code I have so far is:
def db_query(variables):
cursor.execute('SELECT * '
'FROM database.table '
'WHERE id NOT IN (%s)', (variables,))
This does partially work. E.g. If I call:
db_query('593')
It works. The same for any other single value. However, I cannot seem to get it to work when I enter more than one variable, eg:
db_query('593, 595')
I get the error:
psycopg2.DataError: invalid input syntax for integer: "593, 595"
I'm not sure how to enter the query correctly or amend the SQL query. Any help appreciated.
Thanks
Pass a tuple as it is adapted to a record:
query = """
select *
from database.table
where id not in %s
"""
var1 = 593
argument = (var1,)
print(cursor.mogrify(query, (argument,)).decode('utf8'))
#cursor.execute(query, (argument,))
Output:
select *
from database.table
where id not in (593)
I have a data frame in R having 3 columns, using sqlSave I can easily create a table in an SQL database:
channel <- odbcConnect("JWPMICOMP")
sqlSave(channel, dbdata, tablename = "ManagerNav", rownames = FALSE, append = TRUE, varTypes = c(DateNav = "datetime"))
odbcClose(channel)
This data frame contains information about Managers (Name, Nav and Date) which are updatede every day with new values for the current date and maybe old values could be updated too in case of errors.
How can I accomplish this task in R?
I treid to use sqlUpdate but it returns me the following error:
> sqlUpdate(channel, dbdata, tablename = "ManagerNav")
Error in sqlUpdate(channel, dbdata, tablename = "ManagerNav") :
cannot update ‘ManagerNav’ without unique column
When you create a table "the white shark-way" (see documentation), it does not get a primary index, but is just plain columns, and often of the wrong type. Usually, I use your approach to get the columns names right, but after that you should go into your database and assign a primary index, correct column widths and types.
After that, sqlUpdate() might work; I say might, because I have given up using sqlUpdate(), there are too many caveats, and use sqlQuery(..., paste("Update....))) for the real work.
What I would do for this is the following
Solution 1
sqlUpdate(channel, dbdata,tablename="ManagerNav", index=c("ManagerNav"))
Solution 2
Lcolumns <- list(dbdata[0,])
sqlUpdate(channel, dbdata,tablename="ManagerNav", index=c(Lcolumns))
Index is used to specify what columns R is going to update.
Hope this helps!
If none of the other solutions work and your data is not that big, I'd suggest using sqlQuery() and loop through your dataframe.
one_row_of_your_df <- function(i) {
sql_query <-
paste0("INSERT INTO your_table_name (column_name1, column_name2, column_name3) VALUES",
"(",
"'",your_dataframe[i,1],",",
"'",your_dataframe[i,2],"'",",",
"'",your_dataframe[i,3],"'",",",
")"
)
return(sql_query)
}
This function is Exasol specific, it is pretty similar to MySQL, but not identical, so small changes could be necessary.
Then use a simple for loop like this one:
for(i in 1:nrow(your_dataframe))
{
sqlQuery(your_connection, one_row_of_your_df(i))
}