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Get the guideIn BigQuery, the substring function, also known as SUBSTR, is used to extract a portion of a string. The function requires two parameters: the value (the string from which you want to extract a substring) and the position (the point in the string where the extraction should start). An optional third parameter, length, can be used to specify the number of characters to extract.
To construct a query using the substring function in BigQuery, you need to use the SELECT statement and specify the substring function along with the necessary parameters. For example, to extract the first three characters of a string column named 'location', you could use the following query: SELECT SUBSTR(location, 1, 3) AS loc FROM table_name.
SELECT SUBSTR(location, 1, 3) AS loc FROM table_name
The output of the substring function in BigQuery is a new string that contains the extracted substring. For example, if you use the substring function to extract the first three characters of the string 'New York, NY', the output would be 'New'.
The substring function in BigQuery works with STRING or BYTES data types. This means you can use the function to extract a substring from a string or a sequence of bytes.
BigQuery provides several other string functions, including REGEXP_SUBSTR(), which extracts a substring after a character, STRPOS(), which finds the position of a substring within a string, and SPLIT STRING, which returns an array of substrings obtained by splitting the input string at occurrences of the delimiter.
To ensure the substring has been extracted correctly in BigQuery, you can examine the results of your query. If the results match your expectations, the substring has been extracted correctly. If not, you may need to adjust the parameters of the substring function.
When using `SUBSTRING` in BigQuery, users may encounter a few common errors. Here are the five most frequent ones:
1.Invalid function name: `SUBSTRING`
- Error: BigQuery does not use `SUBSTRING`. The correct function is `SUBSTR()`.
- Solution Replace `SUBSTRING` with `SUBSTR()` in your query.
```sql
SELECT SUBSTR(column_name, 1, 5) FROM your_table;
```
2. Incorrect starting index
- Error: BigQuery uses 1-based indexing (the first character is index 1), but many users mistakenly assume 0-based indexing.
- Solution: Make sure to use 1-based indexing.
```sql
SELECT SUBSTR('example', 2, 3); -- starts from the 2nd character
```
3. Negative length
- Error: A negative length in `SUBSTR()` is not valid.
- Solution: Ensure that the length argument is positive or remove the length argument if you want to extract the rest of the string from a starting index.
```sql
SELECT SUBSTR('example', 2, -3); -- incorrect
SELECT SUBSTR('example', 2, 3); -- correct
```
4. Non-integer start or length values
- Error: `SUBSTR()` expects integer values for the start and length, but users sometimes pass non-integer values.
- Solution: Ensure the start and length values are integers.
```sql
SELECT SUBSTR('example', 'two', 3); -- incorrect
SELECT SUBSTR('example', 2, 3); -- correct
```
5. Out-of-bounds index
- Error: If the start index is greater than the length of the string, or if the length value exceeds the number of remaining characters, the result might be unexpected (an empty string or incomplete substring).
- Solution: Ensure the start index and length are within the bounds of the string.
```sql
SELECT SUBSTR('example', 10, 5); -- returns an empty string
SELECT SUBSTR('example', 2, 10); -- returns 'xample'
```
These are the most common errors when using the SUBSTR function in BigQuery, and being aware of them can help you avoid mistakes in your queries.
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