Coalesce Columns in JSON Files Online
Fill null values in JSON files from other columns — return the first non-null value across up to 8 columns in priority order. Add the result as a new column. Runs entirely in your browser.
About this tool
Add a new column to a JSON file containing the first non-null value from an ordered list of up to 8 columns. This is a direct implementation of SQL's COALESCE function. Common uses include building a "best available" value (e.g. use the mobile number if present, otherwise the landline, otherwise the email), merging split address fields, and resolving overlapping data from two sources. The priority order matters — earlier columns take precedence over later ones. Runs entirely in your browser — no upload required.
Frequently Asked Questions
What is the difference between coalesce columns and fill nulls for JSON files?
Fill nulls replaces null values with a static constant (e.g., "0" or "unknown"). Coalesce columns replaces nulls with the value from another column — the first column in your list that is not null. Use coalesce when you want to build a fallback chain across multiple columns.
Does the order of columns matter in the coalesce tool for JSON files?
Yes — order is critical. COALESCE returns the first non-null value in the list. Column 1 is checked first, then Column 2, and so on. If Column 1 has a value it is always used, even if Column 2 also has a value.
What happens if all selected columns are null for a row in a JSON file?
If every column in the coalesce chain is null for a particular row, the result column will also be null for that row. There is no default fallback unless you add a constant as the last column using the Calculate Column tool first.
Is my data private?
Yes — completely. Your file is never uploaded to any server. Everything runs locally in your browser using WebAssembly — processing happens entirely inside your tab. Once you close the tab, nothing is retained.
What is the maximum file size?
The free limit is 50 MB. For larger files, performance depends on your device's available memory — most modern machines handle 500 MB to 1 GB comfortably.
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