Overview
A union table combines several tables into a single series merged by date. The classic use is layering real-time Echo data on top of slower GA4 backfills: the union takes the freshest source for recent days and falls back to the slower source for older ones, so a board never shows a gap while the slow source catches up.
Union is a composite source, so it depends on other tables rather than an external system. When you sync it, the union recomputes from its sources. It does not pull any new external data of its own.
Add sources and set priority
Open Lens > Tables, click + New Table, and choose Union Table under Composite Tables. Name it, then under Source Tables pick a table from the dropdown and click Add. Repeat for each source. The list order is priority order: the table at the top wins when dates overlap. Drag to reorder.
Keep your column names consistent across sources. The union aligns columns by name, so a metric called sessions in one source and session_count in another won't line up. Rename upstream or use Column Labels so the merged table reads cleanly.
Choose a merge mode
Fallback (the default) deduplicates by date. When sources overlap on the same date, the higher-priority source wins and you get one row per date. This is the right choice for the Echo-over-GA4 pattern, where you want a single continuous series with the best available source for each day.
Concat keeps every row from every source. Use it when each source represents a distinct series, for example a multi-line chart that plots one line per source. Fallback's one-row-per-date dedup would collapse those lines, so Concat is what makes long-form multi-line charts possible.
Label columns, save, and sync
If the Column Labels panel appears, set a display name for each column. These labels are used as table headers and chart labels in tiles. Pick a schedule, then Save and Sync. The union recomputes from its sources on each run.
For an Echo plus GA4 blend, put the real-time Echo table at the top of the list and the GA4 table below it, and use Fallback mode. Recent days come from Echo; older days fall back to GA4.
Fallback mode collapses overlapping dates to a single row. If you need every source plotted as its own line, switch to Concat, otherwise the chart will look like one source is missing data.
Troubleshooting
One source seems to be missing from the chart
You're likely in Fallback mode, which keeps one row per date from the highest-priority source. For a multi-line chart that shows each source separately, switch Merge Mode to Concat.
Columns don't line up between sources
The union aligns columns by name. If sources use different names for the same measure, they land in separate columns. Standardize the column names in the source tables, or set Column Labels to present them consistently.
Can't delete a source table
A table can't be deleted while a union depends on it. Remove it from the union's source list first, or delete the union table, then delete the source.