COPY FROM with HEADER

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Overview

When it comes to a table, we deal with its components like rows, columns, and headers. In Oxla, we provide 3 possible options for the header as follows:

• HEADER OFF

This option will not skip the header of the CSV file. Below are the available syntaxes besides HEADER OFF:

HEADER OFF
HEADER FALSE
HEADER 0

• HEADER ON

This option will skip the header of the CSV file and only follow the columns that have been specified before. Below are the available syntaxes besides HEADER ON:

HEADER ON
HEADER TRUE
HEADER 1

• HEADER MATCH

This option will read the header and verify that the name matches the column names.

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Syntax

The syntax for COPY FROM with HEADER is as follows:

COPY table_name FROM 'file_path' (Header_Syntax);

Two parameters need to be specified in the syntax:

• table_name: the table that will receive data from the file.

• file_path: a link to the file location in the server.

• Header_Syntax: the specified header options.

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Examples

Say you have created a CSV file called idvals.csv, and the file has been uploaded to the server:

id,quantity 1,5 2,2 3,1 4,8 5,4 6,3

Then, you create a table by specifying the column with an integer data type:

CREATE TABLE idqty (id INTEGER, quantity INTEGER);

Now, let’s see an example case by case:

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Case #1: HEADER OFF

1. With reference to the table above, run the following query:

COPY idqty FROM 's3://oxla-testdata/cayo/idvals.csv';

2. An error output will appear.

This happens because we specified the table with an INTEGER column. While in the CSV file, we have STRING value which is “id” and “quantity”, which are not considered headers.

Error while parsing f32 from csv at line:0 col:6 position:26, parsing error

Another Option

1. If you don’t want to include the headers (“id” and “quantity”), you can modify your CSV file by deleting the headers:

1,5 2,2 3,1 4,8 5,4 6,3

2. Run the COPY FROM statement:

COPY idqty FROM 's3://oxla-testdata/cayo/idvals.csv';

3. You will get the following output which indicates that the file has successfully imported to the table:

--
(0 rows)

4. Display the table by using the SELECT statement to retrieve the table records:

SELECT * FROM idqty;

+----+----------+
| id | quantity | 
+---------------+
| 1  | 5        |
| 2  | 2        |
| 3  | 1        |
| 4  | 8        |
| 5  | 4        |
| 6  | 3        |
+----+----------+

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Case #2: HEADER ON

1. With reference to the idqty table above, run the following query:

COPY idqty FROM 's3://oxla-testdata/cayo/idvals.csv'(HEADER ON);

2. You will get the following output which indicates that the file has successfully imported to the table:

--
(0 rows)

3. To verify, use the SELECT statement to retrieve the table records:

SELECT * FROM idqty;

We will get the below result, which displays the idqty table:

+----+----------+
| id | quantity | 
+---------------+
| 1  | 5        |
| 2  | 2        |
| 3  | 1        |
| 4  | 8        |
| 5  | 4        |
| 6  | 3        |
+----+----------+

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Case #3: HEADER MATCH

1. Based on the idqty table above, if we run the following query:

COPY idqty FROM 's3://oxla-testdata/cayo/idvals.csv' (HEADER MATCH);

2. It will produce a successful output because the specified columns in the idqty table are matched with the header of the idvals.csv file:

--
(0 rows)

3. But, you will get a mismatched output when the header isn’t matched. Say that the idvals.csv file has “id” and “qty” header, as shown below:

id,qty 1,5 2,2 3,1 4,8 5,4 6,3

Then, you will get a mismatched output because it reads “qty” from the CSV file when the expected value is “quantity” as specified in the table.

column name mismatch in header line field 1: got "qty", expected "quantity"

Another Option

1. Furthermore, you can also define the columns that you want to match as shown below:

COPY idqty(id, quantity) FROM 's3://oxla-testdata/cayo/idvals.csv' (HEADER MATCH);

The following output shows a successful result:

--
(0 rows)

2. But, if you change the ordering by switching the order of the columns:

COPY idqty(quantity, id) FROM 's3://oxla-testdata/cayo/idvals.csv' (HEADER MATCH);

3. You will get a mismatch error message.

column name mismatch in header line field 1: got "id", expected "quantity"