Why there is no phase relationship between the $y$ and $z$ components of electric fields in an unpolarised light The book says:

if light is travelling along $x$ axis, there is no phase relationship between the $y$ and $z$ components of the electric fields in an unpolarised light, 
  my concern is will there be any phase relation between the two in a polarised light, when the $y$ and $z$ components are just the components of the same wave.

How there will be no phase relationship, I believe it should be 0, if there is no relationship it should be between any two waves not with the components of the same wave.
 A: 
my concern is will there be any phase relation between the two in a polarised light,

For example, if the polarization is linear, then the y and z components are in phase (or exactly 180 degrees out of phase). Or possibly one of the components is 0. Or, if the polarization is circular, then there is a 90 degree phase difference between the y and z components.

when the y and z components are just the components of the same wave,how there will be no phase relationship,I believe it should be 0,

If you have 0 phase difference between the y and z components of $\vec{E}$, then you have linearly polarized light with $\vec{E}$ in the direction at 45 degrees between $\hat{y}$ and $\hat{z}$.

if there is no relationship it should be between any two waves not with the components of the same wave.

In order to have unpolarized light, you do need the y and z components to be uncorrelated. This does imply that they are produced by statistically independent processes, and so it would be fair to say they're separate waves. It also generally implies that the process producing the light is incoherent. If it were producing a single frequency, the same for both the x and y components of $\vec{E}$, then there would be some phase relationship between them, even if it were not well controlled. So that means, not only is the y-polarized light a separate wave from the z-polarized light, but also the y-polarized light at $t=0$ is a separate wave from the y-polarized light at $t=\epsilon$, for fairly small values of $\epsilon$.
