I am new to PostgresSQL.I tried
select * from employee where employee_name="elina";
But that results error as follows:
ERROR: column "elina" does not exist.
Then I tried by replacing double quotes with single quotes as follows:
select * from employee where employee_name='elina';
It result fine..So what is the difference between single quotes and double quotes in postgresql.If we can't use double quotes in postgres query,then if any other use for this double quotes in postgreSQL?
Double quotes are for names of tables or fields. Sometimes You can omit them. The single quotes are for string constants. This is the SQL standard. In the verbose form, your query looks like this:
select * from "employee" where "employee_name"='elina';
As explained in the PostgreSQL manual:
A string constant in SQL is an arbitrary sequence of characters bounded by single quotes ('), for example 'This is a string'. To include a single-quote character within a string constant, write two adjacent single quotes, e.g., 'Dianne''s horse'. Note that this is not the same as a double-quote character (").
Elsewhere on the same page:
There is a second kind of identifier: the delimited identifier or quoted identifier. It is formed by enclosing an arbitrary sequence of characters in double-quotes ("). A delimited identifier is always an identifier, never a key word. So "select" could be used to refer to a column or table named "select", whereas an unquoted select would be taken as a key word and would therefore provoke a parse error when used where a table or column name is expected.
TL;DR: Single quotes for string constants, double quotes for table/column names.
Well single quotes are used for string literals and double quotes are used for escaping DB objects like table name / column name etc.
Specifically, double quotes are used for escaping a column/table name if it's resemble to any reserve/key word. Though every RDBMS have their own way of escaping the same (like backtique in MySQL or square bracket in SQL Server) but using double quotes is ANSI standard.
Related
I am new to PostgresSQL.I tried
select * from employee where employee_name="elina";
But that results error as follows:
ERROR: column "elina" does not exist.
Then I tried by replacing double quotes with single quotes as follows:
select * from employee where employee_name='elina';
It result fine..So what is the difference between single quotes and double quotes in postgresql.If we can't use double quotes in postgres query,then if any other use for this double quotes in postgreSQL?
Double quotes are for names of tables or fields. Sometimes You can omit them. The single quotes are for string constants. This is the SQL standard. In the verbose form, your query looks like this:
select * from "employee" where "employee_name"='elina';
As explained in the PostgreSQL manual:
A string constant in SQL is an arbitrary sequence of characters bounded by single quotes ('), for example 'This is a string'. To include a single-quote character within a string constant, write two adjacent single quotes, e.g., 'Dianne''s horse'. Note that this is not the same as a double-quote character (").
Elsewhere on the same page:
There is a second kind of identifier: the delimited identifier or quoted identifier. It is formed by enclosing an arbitrary sequence of characters in double-quotes ("). A delimited identifier is always an identifier, never a key word. So "select" could be used to refer to a column or table named "select", whereas an unquoted select would be taken as a key word and would therefore provoke a parse error when used where a table or column name is expected.
TL;DR: Single quotes for string constants, double quotes for table/column names.
Well single quotes are used for string literals and double quotes are used for escaping DB objects like table name / column name etc.
Specifically, double quotes are used for escaping a column/table name if it's resemble to any reserve/key word. Though every RDBMS have their own way of escaping the same (like backtique in MySQL or square bracket in SQL Server) but using double quotes is ANSI standard.
In a Presto-compatible database (AWS Athena) I have some rows that contain values in double-quotes, mixed with values without double-quotes
e.g. column "postal code" can have "00100" and 00100.
What SQL query can I issue to remove all double-quotes found, so "00100" becomes 00100?
You can use replace():
replace(postal_code, '"', '')
This can be in an update or select.
Note: This removes all double quotes. If you have some values that could have double quotes in them, then you need more complicated expressions. In my experience, though, a column named postal_code would never have double quotes so this is safe.
I have a few column names that have spaces and double quotes in it.
For instance, currently the name of the column is:
Paints the "big picture" of what we aspire to accomplish
If I want to select it how do I do this since there's is double quotes in it?
For other columns I have been doing:
SELECT
"Talks about future trends that will influence how our work gets done"
Which I can't do since I have "big picture" in the column name.
Snowflake appears to permit use of double-quotes in identifiers if they are escaped when referenced:
SELECT "Paints the ""big picture"" of what we aspire to accomplish"
With the caveat that this is only a workaround until you eliminate object names with double quotes -- for any object name wrapped in double quotes, you'll need to wrap them with three double quotes:
create or replace table """MY_TABLE""" ("""MESSAGE""" string);
insert into """MY_TABLE""" select 'Hello, world.';
select """MESSAGE""" from """MY_TABLE""";
The UI preview will not work with a table / column defined this way, and you may encounter other issues. Consider it a workaround until you can change the object names to eliminate double quotes.
This is very simple but somehow I'm doing something wrong with this query on my database.
I have this query below:
SELECT login FROM accounts WHERE login = "loginname";
When I execute this query the result I get is this:
column "loginname" does not exist
This is a no brainer, why is this query not working properly? I have a login column and I know that this user exists because I've found this person with the rails console. Why is the login criteria referring to itself as a column?
Try with single quotes '' if you are trying to match a string
SELECT login FROM accounts WHERE login = 'loginname';
Check the documentation
There is a second kind of identifier: the delimited identifier or
quoted identifier. It is formed by enclosing an arbitrary sequence of
characters in double-quotes ("). A delimited identifier is always an
identifier, never a key word. So "select" could be used to refer to a
column or table named "select", whereas an unquoted select would be
taken as a key word and would therefore provoke a parse error when
used where a table or column name is expected.
Double quotes (") are used to refer to object names, in a case sensitive way. In this case, "loginname" is interpreted as a column name, and the query fails, since there is no such column. In order to refer to a string literal, you should use single quotes ('):
SELECT login FROM accounts WHERE login = 'loginname';
-- Here ---------------------------------^---------^
It seems that the " " are the problem if you believe the documentation. Single quotes are required for string values.
From the PostgreSQL Documentation:
There is a second kind of identifier: the delimited identifier or quoted identifier. It is formed by enclosing an arbitrary sequence of characters in double-quotes ("). A delimited identifier is always an identifier, never a key word. So "select" could be used to refer to a column or table named "select", whereas an unquoted select would be taken as a key word and would therefore provoke a parse error when used where a table or column name is expected. The example can be written with quoted identifiers like this:
UPDATE "my_table" SET "a" = 5;
Quoted identifiers can contain any character, except the character with code zero. (To include a double quote, write two double quotes.) This allows constructing table or column names that would otherwise not be possible, such as ones containing spaces or ampersands. The length limitation still applies.
So in your query "loginname" is the same as having loginname without quotes - it's attempting to refer to a column with that name. To make it a literal string, use single-quotes instead.
What is the difference between single quotes and double quotes in SQL?
Single quotes are used to indicate the beginning and end of a string in SQL. Double quotes generally aren't used in SQL, but that can vary from database to database.
Stick to using single quotes.
That's the primary use anyway. You can use single quotes for a column alias — where you want the column name you reference in your application code to be something other than what the column is actually called in the database. For example: PRODUCT.id would be more readable as product_id, so you use either of the following:
SELECT PRODUCT.id AS product_id
SELECT PRODUCT.id 'product_id'
Either works in Oracle, SQL Server, MySQL… but I know some have said that the TOAD IDE seems to give some grief when using the single quotes approach.
You do have to use single quotes when the column alias includes a space character, e.g., product id, but it's not recommended practice for a column alias to be more than one word.
A simple rule for us to remember what to use in which case:
[S]ingle quotes are for [S]trings Literals (date literals are also strings);
[D]ouble quotes are for [D]atabase Identifiers;
Examples:
INSERT INTO "USERS" ("LOGIN", "PASSWORD", "DT_BIRTH") VALUES ('EDUARDO', '12345678', '1980-09-06');
In MySQL and MariaDB, the ` (backtick) symbol is the same as the " symbol. And note that you can't use " for literal strings when your SQL_MODE has ANSI_QUOTES enabled.
Single quotes delimit a string constant or a date/time constant.
Double quotes delimit identifiers for e.g. table names or column names. This is generally only necessary when your identifier doesn't fit the rules for simple identifiers.
See also:
Do different databases use different name quote?
You can make MySQL use double-quotes per the ANSI standard:
SET GLOBAL SQL_MODE=ANSI_QUOTES
You can make Microsoft SQL Server use double-quotes per the ANSI standard:
SET QUOTED_IDENTIFIER ON
In ANSI SQL, double quotes quote object names (e.g. tables) which allows them to contain characters not otherwise permitted, or be the same as reserved words (Avoid this, really).
Single quotes are for strings.
However, MySQL is oblivious to the standard (unless its SQL_MODE is changed) and allows them to be used interchangably for strings.
Moreover, Sybase and Microsoft also use square brackets for identifier quoting.
So it's a bit vendor specific.
Other databases such as Postgres and IBM actually adhere to the ansi standard :)
I use this mnemonic:
Single quotes are for strings (one thing)
Double quotes are for tables names and column names (two things)
This is not 100% correct according to the specs, but this mnemonic helps me (human being).
Two Things To Remember :
Single Qutoes(') : String Or Text
select * from employees where room_name = 'RobinCapRed';
where RobinCapRed is a string or a text.
Double Quotes(") : Column Names or Table Names
select "first_name" from "employees";
where first_Name is a column name from employees table
The difference lies in their usage. The single quotes are mostly used to refer a string in WHERE, HAVING and also in some built-in SQL functions like CONCAT, STRPOS, POSITION etc.
When you want to use an alias that has space in between then you can use double quotes to refer to that alias.
For example
(select account_id,count(*) "count of" from orders group by 1)sub
Here is a subquery from an orders table having account_id as Foreign key that I am aggregating to know how many orders each account placed. Here I have given one column any random name as "count of" for sake of purpose.
Now let's write an outer query to display the rows where "count of" is greater than 20.
select "count of" from
(select account_id,count(*) "count of" from orders group by 1)sub where "count of" >20;
You can apply the same case to Common Table expressions also.