Notes. N_obj lists the GRB or quasar sample sizes, and N_{Mg ii} is the number of detected absorbers within the total redshift path length, Δz, with an average redshift, ⟨ z ⟩. The penultimate column lists the reported excess of absorbers relative to the expectation from SDSS quasar absorber statistics. One exception is that Bergeron et al. (2011) analysed blazars rather than GRBs. Vergani et al. (2009) and Cucchiara et al. (2013) reported statistics from high-spectral-resolution data, as well as combined high- and low-spectral-resolution data. It should be noted that there are substantial overlaps of the target selections amongst the references, and the reported overdensities are therefore correlated. To compare our searches with the literature, we report statistics from absorbers at different redshift ranges: z< 2.3 and z> 2.3 (Sect. 3.2). The last column reports overdensities relative to high-spectral-resolution quasar studies by Mathes et al. (2017) for the low-redshift sample, and Chen et al. (2016) for the high-redshift sample.