Using 'If else' command in Expression Builder

I am having 'n' number of Variations and want to assign a Text variable to describe each variations.

For that, I am just trying to assign an 'if else' expression to that text variable. 
I can use 'conditional' for an if statement. But, an if statement can be used to test a single condition and return 'true' or 'false'.

I need to run different test conditions in a single argument as I am having many Variations.

For that, I need 'if else' function. But, 'if else' function couldn't be found in expression builder.

For instance,

For column variation '50x50', the variable should be 'ABC123'.

For column variation '75x75', the variable should be 'DEF456'.

For column variation '100x100', the variable should be 'GHI789'.

For column Variation '150x150', the variable should be 'JKL012'.