Combine datasets of unequal lengths by matching dates

My question has 3 sub-questions: I have 10 matrices (10 different variables: V1 V2, V3…, V10) that each one has 2 or 3 columns and a few hundreds of rows (different size each one). The first column at every matrix is the date (serial date number), the second (when there) is the depth and the third is a measured value of a variable (temperature, chlorophyll, etc). 1. I want to create a new matrix that will have all variables combined based on first column (date), and the rest of the columns will be the values of the variables that match these dates. In fewer words, I need to concatenate all matrices by matching up the dates from their first column.

2. I need to do the same as in (1) but not using all the dates recorded. Instead use just the dates of a certain variable (for example 'V1') as the matching point and line up the variables according to the dates of ‘V1’.

3. Lastly, I need to combine the data from the variables that have 3 columns when the elements of the first two elements in each row are the same between the different variables. So, the desired matrix here will have the date and depth as the first 2 columns and then the rest of the columns will be the measurements of the different variables at that day and that depth. So I have to concatenate the matrices by matching the elements of the first 2 columns.

I have tried a few things for each case but nothing seemed to work as desired.

I would appreciate your input to any or all of my sub-questions.

Warm regards, Niki